MINIMIZING THE RISK OF ACCIDENTAL EXPOSURE FROM NEWER RADIATION THERAPY1

TECHNOLOGIESICRP REF: 32/147/07

INTERNATIONAL COMMISSION ON RADIOLOGICALPROTECTIONPREVENTING ACCIDENTALEXPOSURES FROM NEW
EXTERNAL BEAM RADIATIONTHERAPY TECHNOLOGIESDRAFT 8 (FEBRUARY, 2009)TASK GROUP MEMBERS
P. Ortiz (Chairman)
J. M. Cosset
O. Holmberg
J. C. Rosenwald
P. Dunscombe
CORRESPONDING MEMBERS
L. Pinillos
J.J. Vilaragut
S. Vatnitsky

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MINIMIZING THE RISK OF UNINTENDED EXPOSURE FROM NEWER RADIATIONTHERAPY TECHNOLOGIETEN MAIN POINTS
1. The following conclusion for conventional radiation therapy from ICRP 86 iseven more relevant and important for new technologies: “purchasing new
equipment without a concomitant effort on education and training and on a
programme of quality assurance is dangerous”. The lessons from reported
accidental exposure have confirmed that embarking on new technologies makes
revisiting staff qualifications essential. Chapter 3 and 4 of this report identify
critical issues that may require additional training..
2. Increased complexity requires a strategy of combining:•

Manufacturer design of safety interlocks, proper alerts and warnings, selftest capabilities for equipment, easy-to-understand user interfaces in alanguage understandable to the user and adherence to international
standards to ensure compatibility between equipment from different
manufacturers.

All these safety measures are equally important forsoftware.
•Education and equipment specific training with formal involvement of
manufacturers. Such training should address the understanding of warnings
and interlocks provided by manufacturers, in particular those related tobeam monitoring, and consider such warnings and interlocks in developing
acceptance tests as well as in quality control procedures.

•

Risk-informed and cost-effective approaches for prioritizing tests andchecks by means of prospective methods of risk assessment, to be performedin cooperation with manufacturers.

3. Hospital administrators and heads of radiation treatment programs should
provide for an environment in which to “work with awareness”, inviting
concentration and avoiding distraction, and ensuring supervision of compliancewith QA procedures. Reported experience has shown that skipping quality
control checks has occurred even in well structured departments. An attitude oflearning from experience should be cultivated, together with systematic reportingof near misses and unexpected events, which may help to reveal weak points inthe system before an accident occurs. Excessive confidence in computers andMINIMIZING THE RISK OF UNINTENDED EXPOSURE FROM NEWER RADIATION

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THERAPY TECHNOLOGIES

automatic processes may result in relaxing attention to potential pitfalls that may
still occur, even with these systems in place, as identified in sections 3 and 4 ofthis report.
4. Hospital staff should stay aware of the fact that the overall responsibility for thecorrect absorbed dose determination and the correct treatment of patientsremain with the user in hospital. These responsibilities include investigating
discrepancies in dose measurements, before applying the beam to patient
treatments.

Manufacturers

andsuppliers,

however,have

subsidiary

responsibility for delivering correctly functioning equipment with correctcomputer files and accompanying documents, and for providing correct
information and advice, upon request from hospital staff. Independent checks ofmeasurements and calculations continue to be a vital safety layer
5. Target dose escalation without a concomitant increase in normal tissuecomplication probability generally implies a reduction of geometrical margins,which is only possible with conformal therapy accompanied by precise, imageguided

patient

positioning

andeffective

immobilization

and

a

clear

understanding of the accuracy achieved in clinical practice. Without thesefeatures, target dose escalation could lead to severe patient complications.
6. A programme for purchasing, acceptance testing and commissioning should not
only address the treatment machine but also, increasingly, complex treatment
planning systems, “record and verify” systems, imaging equipment used forradiation therapy, software, procedures and entire processes. There is a need forre-commissioning the relevant devices after equipment modifications or software
updates and also to monitor impact on the related processes.

7. Timely and effective sharing of operational experience is critical in the phase of
introducing new techniques and technologies and especially necessary for
problems that appear infrequently, when certain conditions happen to coincide,because such problems may to escape conventional tests.
8. A computer crash may cause loss of data integrity. If it occurs during dataprocessing or data transfer it may be very critical, because regular backup may

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MINIMIZING THE RISK OF UNINTENDED EXPOSURE FROM NEWER RADIATIONTHERAPY TECHNOLOGIE

not help in these situations as shown in this report. Procedures should include asystematic verification of data integrity routinely and particularly after acomputer crash.
9. When introducing new imaging technologies into radiation therapy practice,assessment of radiation doses from imaging becomes necessary. Such doses
should be integrated and accounted for in the treatment planning and delivery
processes. Consistent coordinate systems are required throughout the wholeradiation therapy process. In addition specific procedures are required forrecording image orientation with respect to the patient, for the choosing thecorrect images and CT numbers for tissue density estimation, and for giving
specific consideration to possible image artefacts and possible geometricdistortion.
10. When conventional tests and verification methods are not directly applicable or
not effective for new technologies, measures to maintain the required level ofsafety are needed, even if they entail the design of new tests or the modification
and re-validation of the old ones. In some cases the ‘validation plan’ on aphantom for new sophisticated treatment modalities may be the only major
safety layer. Such verifications should not be relinquished until alternative
checks are in place. Supervision to ensure the maintenance of the required levelof safety is crucial.

MINIMIZING THE RISK OF UNINTENDED EXPOSURE FROM NEWER RADIATION

5THERAPY TECHNOLOGIESCONTENTS
1.

2.

INTRODUCTION ............................................................................................................ 8
1.1.

Background......................................................................................................... 8

1.2.

Trends in radiation therapy................................................................................. 8

1.3.

Trends in risk assessment ................................................................................... 9

1.4.

Objectives of this report ................................................................................... 10

1.5.

Scope ................................................................................................................ 11

1.6.

Structure............................................................................................................ 11

1.7.

References ........................................................................................................ 11

SUMMARY OF LESSONS FROM ACCIDENTAL EXPOSURES WITHCONVENTIONAL TECHNOLOGY............................................................................. 13
2.1.

Organization and quality assurance programme .............................................. 13

2.2.

Special problem of the availability of qualified staff ....................................... 14

2.3.

Safety culture .................................................................................................... 15

2.4.

Lessons from acceptance, commissioning and calibration............................... 15

2.5.

Commissioning of treatment planning systems................................................ 16

2.6.

Treatment related lessons ................................................................................. 17
2.6.1. Treatment preparation........................................................................... 17
2.6.2. Treatment delivery ................................................................................ 18

2.7.
3.

References ........................................................................................................ 19

SAFETY ISSUES WITH NEW TECHNOLOGIES ...................................................... 20
3.1.

Issues of equipment design, acceptance testing and commissioning ............... 20

3.2.

Treatment Prescription ..................................................................................... 22

3.3.

Treatment preparation....................................................................................... 25
3.3.1. Patient immobilization.......................................................................... 26
3.3.2. Patient data acquisition and virtual simulation ..................................... 26
3.3.3. Image segmentation and structure delineation...................................... 28
3.3.4. Beam definition and optimization of the dose distribution................... 29

3.4.

Treatment delivery............................................................................................ 32
3.4.1. Verification of patient and beam setup ................................................. 33

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3.4.2. Intensity Modulated Radiation Therapy and other advanced
dynamic techniques............................................................................... 35
3.5.

Patient data management .................................................................................. 38
3.5.1. Description of existing patient data management systems ................... 39
3.5.2. Machine setup verification functionalities of “recod and verify”
systems.................................................................................................. 40
3.5.3. Electronic chart functionalities of “record and verify” systems ........... 42
3.5.4. Image handling ..................................................................................... 43

3.6.
4.

References ........................................................................................................ 44

REPORTED ACCIDENTAL EXPOSURES WITH NEW TECHNOLOGIES ............ 47
4.1.

Events related to beam output and calibration.................................................. 47
4.1.1. Calibration problems of small-fields .................................................... 47
4.1.2. Intra-operative radiation therapy beam calibration issues .................... 49
4.1.3. Beam output drift in tomotherapy......................................................... 50

4.2.

Issues related to treatment preparation ............................................................. 51
4.2.1. Problems with dynamic wedges ........................................................... 51
4.2.2. Computer problems with intensity modulated radiation therapy.......... 55
4.2.3. Errors in imaging for radiation therapy treatment planning ................. 58
4.2.4. Treatment set up errors from virtual simulation markers ..................... 58
4.2.5. Digitally reconstructed radiograph errors ............................................. 59

4.3.

Events related to patient data management ...................................................... 60
4.3.1. Errors when using “record and verify” systems ................................... 60

4.4.

Events related to treatment delivery and treatment verification....................... 63
4.4.1. Significant radiation exposure from electronic portal imaging ............ 63
4.4.2. Errors with stereotactic radiosurgery field size .................................... 64

4.5.
5.

References ........................................................................................................ 65

PROSPECTIVE APPROACHES TO AVOIDING ACCIDENTAL EXPOSURES ..... 67
5.1.

Treatment process flow diagrams..................................................................... 67

5.2.

Process flow diagram and the design of Quality Assurance programmes........ 69

5.3.

Failure modes ................................................................................................... 70

5.4.

Risk................................................................................................................... 70

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5.5.

Three prospective approaches .......................................................................... 72
5.5.1. Failure mode and effect analysis .......................................................... 72
5.5.2. Probabilistic safety assessment............................................................. 73
5.5.3. Risk matrix............................................................................................ 78

6.

5.6.

Closing the loop and applying prospective methods. ....................................... 78

5.7.

References ........................................................................................................ 81

CONCLUSIONS AND RECOMMENDATIONS ......................................................... 84
6.1.

Complexity ....................................................................................................... 84

6.2.

Dose escalation ................................................................................................. 85

6.3.

Increased role of imaging ................................................................................. 85

6.4.

Increased demands on purchasing, acceptance and commissioning ................ 86
6.4.1. Re-commissioning of modifications and updates................................. 87
6.4.2. Specific safety issues related to treatment planning systems ............... 87

6.5.

Omnipresence of computers ............................................................................. 88

6.6.

Education and training...................................................................................... 89

6.7.

Use of proactive safety assessment in designing a quality assurance
programme........................................................................................................ 89
6.7.1. Change in processes and staff responsibilities...................................... 90
6.7.2. The problem of tests that are no longer effective ................................. 91
6.7.3. Consistency in prescription................................................................... 91
6.7.4. Coordinates, reference marks and tattoos ............................................. 91
6.7.5. Handling of images............................................................................... 92
6.7.6. Uniformity and clarity in data transfer approaches .............................. 92
6.7.7. Safe communication among interdisciplinary professionals ................ 92
6.7.8. Maintenance, repairs and notification of the physicist ......................... 93

6.8.

Safety culture .................................................................................................... 93

6.9.

References ........................................................................................................ 94

APPENDIX A.

SHORT REPORTS ON INCIDENTS FROM THE RADIATIONONCOLOGY SAFETY INFORMATION SYSTEM .................................................... 96

MINIMIZING THE RISK OF UNINTENDED EXPOSURE FROM NEWER RADIATION

8THERAPY TECHNOLOGIE1.

INTRODUCTION
1.1. Background(1)

New technologies have been introduced in radiation therapy, principally aimed at

improving the treatment outcome, by means of a dose distribution which more strictly
conforms to the tumor (clinical target) volume. A highly conformal dose distribution allows
for a dose escalation to the target volume without increase of the radiation dose to normal
tissues. Conversely, a highly conformal dose distribution allows for a reduction of radiationdose to normal tissues without decreasing the dose to the target, or a combination of both.
These new technologies include increased use of multileaf collimators, intensity modulated
radiation therapy (IMRT) and arc therapy (IMAT), tomotherapy, image guided radiation
therapy (IGRT), gated radiation therapy, radiosurgery, newer and more complex treatment
planning systems, virtual simulation and “all inclusive” electronic information systems.
1.2. Trends in radiation therapy(2)

The development of these new technologies has been impressively fast during thelast few years. As an example, 10 years after a new concept for the delivery of dynamic
conformal radiotherapy was described (Mackie et al. 1993), called tomotherapy, a machine
was put into clinical operation, and five years after the first clinical prototype, more than 200
tomotherapy machines are operating throughout the world. Although most machines arelocated in North America and Western Europe, a significant number of such machines are
installed in Asia and the technology is also reaching the Middle East.
(3)

Similar trends are observed for other rotational approaches using computer driven

intensity modulation. These are being implemented on more “traditional” accelerators and areclassified as “Volumetric Arc Therapy” techniques (VMAT) with the trade name dependingon the manufacturer. Other recent developments include on-line volumetric imaging (ConeBeam Computed Tomography with kV or MV beams), robotic solutions (for example the
Accuray© machine), particle therapy with proton or ion beams or additional devices based on“Information Technology” (Treatment Planning Systems, “Record and verify” systems).
(4)

There is therefore a continuous evolution of what is considered a “standard”

machine for radiotherapy, towards more sophisticated equipment which, in turn, requires
more automation to remain efficient. This “standard” varies widely throughout different parts

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THERAPY TECHNOLOGIESof the world. For the high income countries (of North America and Western Europe), the“standard” has become, in the last five years, an accelerator equipped with a multileaf
collimator and a flat panel portal imaging device, integrated with a “record and verify”
device. More recently, an accelerator with intensity modulated radiation therapy (IMRT) has
become the “standard” for many countries.
(5)

While a manual cobalt machine combined with a 2D treatment planning system

was still considered until recently as a good option in the developing countries, there is also atrend in such countries towards more sophisticated solutions with the acquisition of thenecessary equipment. It is difficult to predict at which rate the new technologies will be taken
up over the countries worldwide. However, there is a danger of not ensuring the availability of
appropriate technical, scientific and medical resources for the safe introduction of these new
approaches. The risk of accidental exposure as a consequence of this evolution in technology
should be reduced by appropriate efforts on education as underlined in the present report.
1.3. Trends in risk assessment(6)

Minimizing the risk of accidental exposure to radiation therapy patients haslargely been based so far on compliance with regulatory requirements, codes of practice andinternational standards, which can be considered a “prescriptive approach”. Compilations oflessons learned from the review and analysis of accidental exposures in radiation therapy havebeen published (IAEA 2000, ICRP, 2000). These lessons can be used in safety assessments,for example by checking whether a given radiation therapy department has sufficient
provisions in place to avoid accidental exposures similar to those reported. As an example,
major accidents caused by errors in calibration and commissioning of radiation therapy
equipment have led some departments to put in place preventive measures such as anindependent determination of the absorbed dose to detect possible beam calibration errors.
However, most existing reports relate to conventional radiation therapy as practiced prior to
the widespread implementation of computerized patient data management systems, intensity
modulated radiation therapy and other newer techniques.
(7)

There is a need to assess to what degree lessons learned from conventional

techniques can be also applied to newer more advanced and complex radiation therapy.
Further, there is also a need to find out if there are new lessons to learn through a review of

MINIMIZING THE RISK OF UNINTENDED EXPOSURE FROM NEWER RADIATION

10THERAPY TECHNOLOGIE

recently available information on accidental exposures and near misses resulting from the useof new technologies.
(8)

Approaches to the avoidance of accidental exposures have, to date, largely been“reactive” or “retrospective”. The limitation of such approaches is that of being confined toreported experience, i.e., they do not address the question of “what else could go wrong?” or“what are other potential hazards?” Thus latent risks from other possible events, which have
not yet occurred or have not been published, will remain unaddressed, unless more “proactive
approaches” are applied. In addition, the increasing complexity of new technologies andtechniques means that “common sense” and intuition may no longer be as effective as a guard
against accidental exposures. The ability to perceive “when something may be wrong” isdiminished compared with the situation in conventional radiation therapy. For example, in aconventional two-to-four field technique, it was feasible for someone with experience todiscover an error in dose calculation, by looking at the treatment time or monitor units (MU)
and making relatively simple calculations to verify the monitor unit calculation to within a
few percent.. This is no longer possible in intensity modulation radiation therapy (IMRT).
Increased complexity requires a different strategy to deal with risk and new, selective, riskinformed, and cost-effective approaches to safety and the maintenance of quality are required.
(9)

Prospective approaches to the identification and analysis of failure modes,

assessment of their frequency and consequences and their evaluation in terms of risks, areavailable and are being used by some health care institutions to provide risk-informed
strategies. Such approaches have started to be adopted by the radiation therapy community(Vilaragut, 2008, Duménigo, 2008, Ortiz, 2008, Huq 2007, 2008).
(10)

In summary, both “retrospective” and “prospective” approaches are needed if theintroduction of new technologies and techniques is to enhance the quality of patient treatment
without compromising safety.
1.4. Objectives of this report(11)

The objectives of this report are both to summarize lessons from experience todate and to provide guidance on proactive approaches to the reduction of risk of accidental
exposure in radiation therapy, with emphasis on the use of advanced and complex planningand delivery technologies and techniques.

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1.5. Scope(12)

The current report focuses on external beam radiation therapy. As there are recent

ICRP publications devoted to high dose rate brachytherapy and to permanent implants, thisreport does not deal with these techniques specifically. However, the approaches discussed
here will also find application in reducing the risk of accidental exposure in brachytherapy.
1.6. Structure(13)

In Chapter 2, lessons learnt from conventional radiation therapy are summarized

and discussed in the context of their applicability to the new technologies being introduced
into the clinic. In Chapter 3, a review of new technologies and their implications for safety arepresented. Reported case histories of accidental exposures and near misses when using newtechnologies and the lessons learnt from them are provided in Chapter 4. Three proactive
approaches to enhancing safety are described in Chapter 5. These approaches enable theprioritisation of activities aimed at reducing the frequency of occurrence of errors and theirseverity and optimizing the quality assurance program so that errors may be detected before
impacting clinical treatment. A recapitulation of lessons learnt and recommendations are
given in Chapter 6. A larger sample of case histories are provided in the appendix.
1.7. References
Duménigo, C., Ramírez, M.L., Ortiz López, P.,

McDonnell, J.D., Vilaragut, J.J.,

Papadopulos, S., Pereira, P.P, Gonçalves, M, Morales, J, Ferro, R, López Morones, R,
Sánchez, R., Delgado, J.M., Sanchez, C., Larrinaga, E., Somoano, F., Guillén, A.,
Rodríguez, M. 2008. Risk analysis methods: their importance for safety assessment ofpractices using radiation. XII Congress of the International Association of Radiation
Protection, IRPA 12.
Huq, S. ASTRO, AAPM and NCI. (2007) Quality Assurance of Radiation Therapy andChallenges of Advanced Technologies Symposium. A Method for Evaluating QA needs inRadiation Therapy.
Huq, M.S., Fraass, B.A., Dunscombe, P.B., et al. (2008) A method for evaluating quality
assurance needs in radiation therapy. Int. J. Radiat. Oncol. Biol. Phys. 70, S170-S173

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MINIMIZING THE RISK OF UNINTENDED EXPOSURE FROM NEWER RADIATIONTHERAPY TECHNOLOGIEIAEA 2000, International Atomic Energy Agency. Lessons learned from accidental exposurein radiotherapy. Safety Report Series, No. 17. IAEA, Vienna.
ICRP 2000, International Commission on Radiological Protection, Prevention of accidental
exposure in radiation therapy
Mackie, T. R.; Holmes, T.; Swerdloff, S.; Reckwerdt, P.; Deasy, J. O.; Yang, J.; Paliwal, B. &Kinsella, T. (1993), 'Tomotherapy: a new concept for the delivery of dynamic conformal
radiotherapy.', Med Phys 20(6), 1709--1719.
Ortiz López, P., Duménigo, C., Ramírez, M.L.,McDonnell, J.D., Vilaragut, J.J., Papadopulos,
S., Pereira, P.P., Gonçalves, M., Morales, J., Ferro, R., López Morones, R., Delgado J.M.,
Somoano, F., Álvarez, C., Guillén, A., Rodríguez, M. 2008. Risk analysis methods: Their
importance for the safety assessment of radiotherapy. Annual Congress of the European
Society of Therapeutic Radiology and Oncology (ESTRO).
Vilaragut Llanes, J.J., Ferro Fernández, R., Rodriguez Martí, M,, Ortiz López, P., María Luisa
Ramírez, M.L., Pérez Mulas, A Barrientos Montero, A., Fernando Somoano, F., Delgado
Rodriguez, J.M., Papadópulos. S., Pereira Jr, P.P., López Morones, P., Larrinaga Cortina,
E., Rivero Oliva, J.J., Alemañy, J. Probabilistic Safety Assessment (PSA) of theradiotherapyradiation therapy treatment process with an Electron Linear Accelerator(LINAC) for Medical Uses. XII Congress of the International Association of Radiation
Protection, IRPA 12.

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2. SUMMARY OF LESSONS FROM ACCIDENTAL EXPOSURES WITHCONVENTIONAL TECHNOLOGY
2.1. Organization and quality assurance programme(14)

Publication ICRP 86 (ICRP 2000) points out that most severe accidental

exposures occurred in radiation therapy departments where a Quality Assurance (QA)
programme was not in place, or if it existed, it was not fully implemented and/or verifications
were omitted. Weaknesses identified from accidental exposures with conventional technology
are: insufficient education and training, including poor understanding of the physics of thetreatment equipment and treatment planning systems, absence of appropriate acceptance andcommissioning procedures, misunderstanding instructions for users, verbally communicatedinstructions, omission of some of the checks of the quality control programmes, changing aprocedure without validation, resuming treatments after a major repair without notifying the
responsible person for beam verification, poor notification of unusual tissue reactions andpoor patient follow up.
(15)

As stated in publication ICRP 86, a systematic programme of quality assurance

can detect systematic errors and decrease the frequency and size of random errors.
Minimizing the probability of occurrence and severity of accidental exposures can beachieved with reasonable effort and expense in a radiation therapy department when “twoconditions are fulfilled: i) a comprehensive and coherent quality assurance programme is inplace and ii) some in-vivo dose measurements are performed”.
Recommendation(16)

Hospital managers need to put in place a quality assurance programme that

addresses education, training and continuous professional development, assessment of therequired number and qualification of staff, proper assignment of duties and responsibilities ofqualified staff, organizational structure, written procedures and supervision of compliance.
Procedures should include equipment purchasing, acceptance and commissioning, QA andperiodic QC, use and maintenance and communications over the whole treatment process,patient observation and follow up of abnormal tissue reactions. Particularly important is theregular re-assessment of the number and qualification of staff as workload increases, new

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equipment is purchased and new techniques are taken up into the radiation therapy
programme.
2.2. Special problem of the availability of qualified staff(17)

In many parts of the world, especially in developing countries, the lack ofqualified staff, which is crucial to safety, remains an unresolved issue. This problem affects
radiation oncologists, medical physicists, technologists, dosimetrists and maintenanceengineers. In particular, medical physicists, responsible for calibration of radiation beam andsources, dosimetric treatment planning and radiation physics aspects of quality assurance, areunavailable in many countries. The reasons for this shortage are twofold: firstly, there may be
no programme of education and hands-on training established in the country and it may noteven be feasible to maintain such an educational programme at a national level, because onlya small number of professionals is required. Moreover, in many countries the profession of
medical physics is not formally recognized and, as a consequence, appropriate candidates can
not be attracted. Secondly, sending professionals abroad for education and hands-on training
often results in losing them permanently because they then prefer to stay in the country where
they have been trained. This is particularly true if the profession is not even formallyrecognized in their own countries, as a professional working in the health sector (amongparamedical staff).
Recommendations(18)

Governments need to be aware of these difficulties when embarking on or

maintaining a radiation therapy programme and to make provisions for a system of education,
training (in the country or abroad), have in place a process of certification to formallyrecognize medical physics staff as health professionals and to put in place a programme toretain the staff that are essential in maintaining safety.
(19)

The general recommendations from conventional radiation therapy summarized

above are also valid for new technologies, although some of them, such as in-vivo dose
measurements, may not be suitable for IMRT or may need further development in order forthem to become suitable.

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2.3. Safety culture(20)

A number of reported accidental exposures have been linked to inattention andlack of unawareness of signs that indicated that “something was going wrong”, e.g.
conflicting signals, displays or messages, omission of follow-up of equipment malfunctions or
false alarms. General unawareness was a common feature of most of major accidental
exposures. As an example of lack of awareness, it is common to provide proper shielding forworkplaces to comply with regulations, but it is less common to design the working
environment in such a way that the control panel and patient monitoring devices during
irradiation are located at a place that is free of distraction of the staff. The existence of aquality assurance programme is essential, but even many double checks may be renderedinefficient if the staff function “mechanically” without due thought.
Recommendation(21)

The basic principles of safety culture are of paramount importance to prevent

accidental exposure in radiation therapy. Good practice is necessary but not sufficient.
Avoidance and detection of errors require going beyond good practice, since even a welldesigned system of controls and verification can suffer degradation with time if not well
implemented or parts are omitted or jeopardized. Radiation therapy should be performed withfull knowledge, due thought, mindfulness, alertness and a proper sense of accountability.
Hospital administrators and the heads of radiation therapy departments are responsible forcultivating these qualities and attitudes, and for encouraging excellence, particularly inmatters related to safety. In particular, they need to provide for a working environment that
invites concentration, avoids distraction, and promotes a questioning and learning attitude by
the staff.
2.4. Lessons from acceptance, commissioning and calibration(22)

The absorbed dose calibration is determined at the commissioning stage or after arepair that may affect the beam. There are a number of opportunities for errors in thedetermination of absorbed dose or dose rate calibration, which can lead to under- or over-doseof all treatments as a consequence of the incorrect absorbed dose value. Thus, when thedeviation is large enough to cause death or severe complications, such accidental exposure
may be of a catastrophic nature. Potential errors may be related to misplacing the ionizationMINIMIZING THE RISK OF UNINTENDED EXPOSURE FROM NEWER RADIATION

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chamber, misunderstanding or misreading the ion chamber calibration certificate,misreporting the irradiation parameters used in the calibration, temperature and atmosphericpressure corrections, errors with any of the series of correction coefficients, or simply an error
in the calculation. Case histories of this type of accidental exposure are given in the appendix.
In addition, it is also possible that the absorbed dose is correctly determined, but incorrectly
introduced in the treatment planning system; this type of accidental exposure is described
separately under 3.3.1.
Recommendations(23)

The number of potential errors in dose calculations can be significantly reduced

by using well proven spreadsheets based on widely accepted protocols (IAEA, 1997, 2000),
but misuse cannot be entirely prevented.
(24)Errors can be detected by an independent absorbed dose determination. If two

different persons come to nearly the same result (i.e., <5%), the probability of a major
undetected error is extremely low, because it would require both persons to commit an errorof the same magnitude and in the same direction. Both absorbed dose determinations should
be really independent, i.e., they should not influence each other to avoid repeating the sameerror. One example of independence is the use of a postal TLD audit, provided thatcommencing patient treatment can wait until the TLD results are available. These safetymeasures are applicable to new technologies as well.
2.5. Commissioning of treatment planning systems(25)

Poor understanding of treatment planning systems (TPS) has led to severereported accidental exposures involving large numbers of patients. Mistakes included entering
the wrong basic data on which treatment doses will rely, such as absorbed dose at thereference point, depth doses, dose profiles, and wedge factors. Other types of mistakes
included duplicating a correction for distance and for wedge factors, not being aware that the
TPS already had included these corrections in the calculation of the treatment dose or monitor
units. A simple type of error consisted of using the wrong decay rate (half life) or the wrong
date associated with the initial source activity or initial absorbed dose rate.

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Recommendations(26)

Formal purchasing agreements should include provisions to ensure that

manufacturers provide training for the staff to become fully acquainted with the new systembefore the acceptance is completed. Systematic commissioning of a TPS is as important as thecommissioning of the treatment machine. There are well recognized international protocols
that can be used as guidance (IAEA, 2007, 2008) for the tests to be performed during both
acceptance testing and commissioning of a TPS. The proper use should never rely upon verbal
transmission of procedures or prescriptions. Instructions for use should be in a language
understandable to the users. All of these lessons are also applicable to new technologies.
2.6. Treatment related lessons
2.6.1.
(27)

Treatment preparation
One of the major reported accidental exposures was related to a change in the useof the TPS, deviating from the normal procedures without validating the new procedures, andwithout independent calculation of the dose to a point for each patient.
(28)

Nowadays, independent calculation of dose to a point or monitor units (MU) is

done using a home-made spreadsheet or with the commercial MU calculators. There havebeen accidental exposures due to transfer of home-made sheets to others without proper
understanding by the recipient. Mistakes were also made at treatment simulation due toincorrect labeling of the images resulting in treating the wrong side of the patient.
Recommendations(29)

Deviations from manufacturer instructions for use should be avoided, and when

this is not possible, they should be thoroughly discussed with the manufacturer and subject tospecific tests and validation before use for clinical treatments. Calculation of the dose to apoint or monitor units (MU) for each patient independent from the TPS would have avoided
most of the major accidental exposures. Quality assurance needs to be applied to home-madespreadsheets for MU calculations and to any commercial MU calculators. This is even more
important with transferring the home-made spreadsheets to others.
(30)

These recommendations are also applicable to new technologies, although theindependent calculation of MU may no longer be as simple as with conventional techniquesand may not even be feasible in the case of IMRT.

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THERAPY TECHNOLOGIE(31)“In vivo” dosimetry can also detect deviations in dose from that prescribed at theentrance or exit of the beam. These deviations may arise not only from the determination ofabsorbed dose at a reference point but also from errors in the calculation of treatment doses
and in treatment set-up. Unfortunately, “in-vivo” dosimetry is neither suitable nor
straightforward for some new technologies such as IMRT.
2.6.2. Treatment delivery(32)

Reported errors during treatment set up and delivery were related to treating thewrong patient, the wrong anatomical site, or using the wrong dose. There was a variety ofreasons including using the wrong patient’s chart without proper identification (i.e. by aphotograph or similar method), confusing fiducial marks and tattoos, different patient
positioning for simulation and treatment, wrong selection of parameters, for example,
machine set for rotation therapy rather than stationary treatment, failure to realize that thetreatment of one of the sites was already completed and failure to introduce intended wedges.
Recommendations(33)

Identification of the patient, the treatment site and plan is essential. Provision for

identifying the patient by a photograph is indispensable as is a provision for identifying
fiducial markers and tattoos. Modern digital techniques make this approach simpler, as every
radiation therapy department can have digital cameras and the means to incorporate thepicture into the treatment chart. Modern techniques can also be useful to further ensure
identification in future, such as individual identity card with bar code or fingerprint
identification. Some of the positioning errors are automatically excluded by “record andverify” systems, although these systems can bring other type of problems such as relying too
much on an automatic system as opposed to a manual system where the user is forced tomaintain a higher degree of vigilance.
(34)

Patient set-up mistakes can be avoided or detected by independent checks by two

technologists. To make this task efficient it may be necessary to do a prior identification ofcritical safety steps to be double-checked and the appropriate instructions to be given to thetechnologists.

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2.7. References
IAEA International Atomic Energy Agency (1997). Absorbed Dose Determination in Photon
and Electron Beams: An International Code of Practice, IAEA-Technical Report SeriesNo. 277, Second Ed., IAEA, Vienna (1997).
IAEA International Atomic Energy Agency (2000). Absorbed Dose Determination in External
Beam Radiotherapy: An international Code of Practice for dosimetry based on standardsof absorbed dose to water, IAEA Technical Report Series 398, IAEA, Vienna (2000).
IAEA (2007), Specification and Acceptance Testing of Radiation therapy Treatment Planning
Systems. IAEA TECDOC-1540. Vienna, Austria.
IAEA (2008), Commissioning of Radiotherapy Treatment planning Systems: testing for
typical external beam treatment techniques. IAEA TECDOC-1583. Vienna, Austria
ICRP (2000) International Commission on Radiological Protection.. Prevention of accidental
exposure of patients undergoing radiation therapy, ICRP Publication 86. Annals of the
ICRP 30(3), (Pergamon Press, Oxford).

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3.

SAFETY ISSUES WITH NEW TECHNOLOGIES

3.1. Issues of equipment design, acceptance testing and commissioning•

Recent advances in radiation therapy have only been achievable by increasing
complexity. As a result of complexity, “human common sense” may no longer be aseffective a mechanism to perceive “when something may be wrong” as it is withconventional radiation therapy. The challenge is, therefore, to implement complex
technologies in conjunction with the appropriate means to ensure that they can andwill be used safely.

•

The most important feature related to the complexity and sophistication of “newtechnologies” is the omnipresence of computers. There is a need for software design
with proper warnings, self-test capabilities, self-explanatory user interfaces andinternal safety interlocks to prevent improper use that may lead to an accidental
exposure. Specific training becomes a major issue for users as well as for
manufacturers and installation and maintenance engineers.

•

A number or reports on accidental exposures with new technologies and near misses
is given in chapter 4 and in the appendix, but there is a need for anticipating criticalissues from the safety point of view. This chapter is an attempt to such anticipation,although more detailed proactive analysis is required, as explained in chapter 5.
(35)

The introduction of new technologies in radiation therapy is principally aimed at

improving the treatment outcome, by means of a dose distribution which more strictly
conforms to the tumor (clinical target) volume. A highly conformal dose distribution allows
for a dose escalation to the target volume without an increase in the radiation dose to normal
tissues, or for a decrease in normal tissue dose without reducing tumor dose, or a combinationof both. Such refinement of the dose distribution actually delivered to the patient may beobtained through the following :• providing technical solutions on the irradiation equipment to improve the dose
distribution conformity to the target (beam intensity modulation combined or notwith gantry rotation, multibeam approaches and hadrontherapy for example)• providing treatment planning tools to optimize the dose distribution for each of these
new technical solutions (inverse planning approach)• providing the means to apply them accurately to individual patients (image guided

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radiation therapy and breathing management techniques for example)(36)

Since most recent advances and improvements in radiation therapy are only

achievable by increasing complexity in both equipment and treatment techniques they may
also increase the number of opportunities and scenarios in which an accidental exposure may
be triggered. In order to achieve the desired clinical outcome improvements without anincrease in risk, safety provisions and strategies are necessary. The challenge is, therefore, toimplement new technologies in conjunction with the appropriate means to ensure that they
can and will be used safely.
(37)

The most important feature related to the complexity and sophistication of “newtechnologies” is the omnipresence of computer based solutions. Computers are used at eachstage of the process, from prescription to completion of the treatment. As a result of thiscomplexity, “human common sense” may no longer be as effective a mechanism to perceive“when something may be wrong” as it is with conventional radiation therapy (Rosenwald2002). In a conventional two-to-four field technique, it is feasible for someone withexperience to identify an error, by looking at the treatment time or MU and making some
simple calculations to confirm that these machine settings will result in the delivery of thecorrect dose to within a few percent. This is no longer feasible in IMRT for example in which,instead of a collimator with four jaws and a relatively simple control mechanism, multileaf
collimators are generally required and these have 80 or more computer-controlled leafs, used
to generate many irregular fields applied either as a descrete sequence (step and shoot mode)or dynamically (sliding window or rotational modulation).
(38)

A strategy for the user to systematically deal with risks from complex procedures

will be discussed in Chapter 5. In addition, the role of the manufacturers is of increasing
importance.

As far as software is concerned, there is a need for the design of proper

warnings, self-test capabilities, self-explanatory user interfaces and internal safety interlocks
to prevent improper use that may lead to an accidental exposure. Specific training becomes amajor issue for users as well as for manufacturers and installation and maintenance engineers.
(39)

The normal process when introducing a new or upgraded piece of equipment orsoftware component in a radiation therapy department starts with planning, followed by thepurchase, installation and acceptance. Acceptance is the process in which the new item istested with respect to predefined specifications agreed upon with the vendor and reflected in
the purchase contract. Upon acceptance, the new item is declared compliant with the purchaseMINIMIZING THE RISK OF UNINTENDED EXPOSURE FROM NEWER RADIATION

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order which means that the vendor may be paid and that the warranty period may start.
However, before the equipment or software is used clinically, it is essential to complete a
commissioning phase that requires significant time and effort. The commissioning phaseincludes the calibration and customization of the system for clinical implementation at theuser’s site.
(40)

Specific training on the particular equipment to be used is initiated as part of thepurchase and acceptance process but it needs to be completed during the commissioning
phase in order to ensure that all staff that will use the equipment or software has become
familiar with it and its associated safety features and has developed the required expertise forits safe and effective use. If a calibration or basic data input error occurs during thecommissioning phase it will be potentially propagated to all patients either planned or treatedwith this hardware or software component. New technologies are not different in this respectfrom conventional technologies but the increase in complexity entails new challenges when
designing comprehensive commissioning programmes intended to detect any potential
pitfalls. Risk-informed strategies to rationally deal with this problem are given in chapters 4
and 5.
(41)

The steps of the treatment process are related to individual patients and each steppresents associated risks. The major broad steps are:• treatment prescription• treatment preparation• treatment delivery(42)

These steps are integrated into a workflow that makes extensive use of computerresources and requires patient data management. Patient data management is also identified
as one cause of additional risk and has a separate section in this chapter. The main additionalrisks derived from the introduction of new technologies are grouped under these items.
3.2. Treatment Prescription•

The dose distribution from intensity modulation radiation therapy (IMRT) may
exhibit a less uniform dose distribution in the target than conventional approaches.
This makes conventional dose reporting at a single specific point (the so called“ICRU reference point”) somewhat hazardous and, therefore, no longer acceptable.

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THERAPY TECHNOLOGIES•IMRT and inverse planning require significant changes in the approach to doseprescription which now needs to be expressed in terms of dose-volume objectives(e.g. minimum and maximum doses within the target volume) and dose-volume
constraints (i.e. maximum doses to specified volumes of the organs at risks).

•

Dose escalation with associated geometrical margin reduction is only acceptable inconjunction with an improvement in the dose conformality, accompanied byaccurate and sophisticated imaging methods to check and monitor patient
positioning. If this requirement is not fully appreciated, there is a risk thatprescribing dose escalation could lead to severe patient complications.
(43)

Treatment prescription is the responsibility of the radiation oncologist. It consistsof choosing a therapeutic dose to be delivered to a given target volume according to a given
time pattern or fractionation. The acceptable dose to organs at risk is also part of theprescription. The nomenclature and definitions of the various components of the prescription
need to be standardized, in order to avoid misinterpretation of the prescription within a single
institution or when sharing experience among several institutions. This standardization has
been facilitated by the International Commission on Radiation Units and Measurements(ICRU) which has given recommendations about volume definition and dose reporting for“conventional” treatments (ICRU 1993, 1999).
(44) The development of intensity modulated radiation therapy (IMRT) and inverse planninghas necessitated significant changes in the approach to dose prescribing which now needs tobe expressed in terms of dose-volume objectives (e.g. minimum and maximum doses within
the target volume) and dose-volume constraints (i.e. maximum doses to specified volumes ofthe organs at risks).
(45)

The dose distribution in the target for an IMRT plan can be considerably lessuniform than that achieved with conventional approaches. This makes conventional reportingat a single specific point (the so called “ICRU reference point”) somewhat hazardous and,
therefore, no longer acceptable.
(46)

Clear recommendations at the national or international level and strict applicationof local protocols, are indispensable to avoid inconsistency between treatments given within asingle institution or in different institutions. As an example, Das et al. (Das 2008) havereported large deviations between prescription and calculated doses for 803 patients treatedwith IMRT at five medical institutions (see Fig. 2.1, below). Part of this deviation may be

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attributed to a lack of consistency in the prescription and/or reporting practice. Depending on
the treatment planning system (TPS), the user may be tempted to prescribe in different forms
and would not necessarily realize that the same intended prescription, if expressed in different
forms, would yield different outcomes. Although such a deviation is not per se an accidental
exposure, the associated risk should be recognized.

Fig. 2.1 taken from Das et al. JNCI, March 2008
Figure 2.1 illustrates the deviation between the prescribed dose and the dose calculated
retrospectively for 803 patients treated with IMRT at five medical institutions, where different
treatment planning systems were used. The calculated dose is indicated either at the isocenter(the ICRU reference point for most treatments) as represented by the blue triangular symbols,or as median, minimum or maximum dose within the target volume. For most patients themedian dose is close to the prescribed dose (96% lie within ± 10%) but the dose at theisocenter shows much greater deviation since the isocenter might be in a high gradient region
for some of the modulated beams This suggests that applying the ICRU reference point
concept for IMRT prescription could lead to significant spread of the delivered dose amongst
the patient population (Das et al. INCI, March 2008).

(47)

An additional risk, which can accompany more advanced techniques, is related todose escalation to the target volume, in order to achieve a better tumour control probability(TCP), while keeping an acceptable normal tissue complication probability (NTCP). ThisMINIMIZING THE RISK OF UNINTENDED EXPOSURE FROM NEWER RADIATION

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requires an improvement in the dose conformality, as achieved by a reduction of geometricalmargins, accompanied by accurate and sophisticated imaging methods to check and monitor
patient positioning. If this requirement is not fully appreciated, there is a risk that prescribing
dose escalation could lead to severe patient complications.
3.3. Treatment preparation•

With the increased use of different imaging modalities in radiotherapy, consistency
becomes a major safety issue: there is a need for consistency in the coordinate systemfrom treatment preparation to delivery, in patient identification and in image
orientation with respect to the patient. CT image artifacts become a potential sourceof error when CT numbers are converted into tissue densities. The need for
integration of radiation exposure from imaging into the treatment plan also needs tobe considered.

•

Increased TPS complexity poses a challenge and requires understanding of its
multiple functionalities, understanding of its limitations in anatomical modeling,
adherence to the instructions for use of the system (avoiding changes in the way the
TPS is used without proper testing and validation). The design and testing forsoftware warnings and interlocks is becoming increasingly important.
(48)

Treatment preparation consists of all the tasks to be performed before the actual

radiation delivery is initiated. Treatment preparation is also sometimes called “treatment
planning” in a broad sense – see e.g. Fraass et al. 1998 and IAEA 2004 and includes thefollowing steps :1. patient immobilization
2. patient data acquisition, combined or not with virtual simulation 1

1

The conventional “simulation” done on a simulator, used to be a radiological verification of beam set-up. Thistask would normally fit between step 4 and 5. Conventional simulation tends to be replaced with the “virtual
simulation”, in which the beams are defined on a computer console (possibly in real time) on the basis of the 3D
reconstruction of the patient anatomy. This is sometimes combined with step 2 although covering partly steps 3
to 4,.

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3. image segmentation and structure delineation
4. beam definition and optimization of dose distribution (sometimes called “doseplanning”)(49)

The associated workflow through these steps requires networking and dataexchange between, devices which may be from different manufacturers and which require fullinterconnectivity and interoperability. The risks related to the process itself will be dealt within Chapter 5. In what follows, issues related to the equipment used at each individual step of
the treatment preparation process and the accompanying procedures are addressed.
3.3.1.
(50)

Patient immobilization
Properly immobilizing the patient to help in interfraction reproducibility and in

reducing intrafraction motion is the first step of the treatment planning process.
Immobilization has always been difficult in radiation therapy. Modern immobilization
techniques do not present major risks in terms of excessive temperature or chemical toxicity.
The most severe risk to consider might be the restriction of vital patient movements (e.g.
breathing normally, vomiting in the case of nausea and avoiding a collision). It should bepossible for the patient to immediately warn the operator if anything abnormal occurs, either
during treatment preparation or beam delivery. A possibility is to give to the patient access toa push button alarm. This is done for patients treated with automatic breathing control (ABC),
where a valve inhibits patient respiration when the beam is on.
(51)

The need for more accurate patient positioning may encourage people to use

more constraining patient immobilization devices and may therefore require more attention to
the associated risk. On the other hand, the development of image guided radiation therapy(IGRT) may provide more flexibility due to the possibility of correcting, in real time, any
patient misalignment with respect to the treatment beams.
3.3.2.
(52)

Patient data acquisition and virtual simulation
Most modern treatment plans are based on CT data. More and more, otherimaging modalities are added to provide more accurate delineation of the target volumes andorgans at risk. Virtual simulation, which follows patient data acquisition, is becoming
increasingly popular. It can be performed when the patient is still present (which allows skinmarking of the simulation beam projections) or after the patient has left (using a coordinate

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system referenced to radio-opaque markers present during image acquisition and referring therepositioning for simulation with respect to this system).
3.3.2.1.
(53)

Patient orientation for imaging
The CT data acquisition procedure as such does not bring significant additionalrisks. However, with the increased use of CT images, identification of the patient and, more
importantly, of patient orientation becomes crucial. Although the most common patientorientation is “head first–supine” (HFS), it could be necessary to use other orientations, forinstance having the patient prone to treat the spinal cord (medulloblastoma) or “feet first” totreat a leg. In any case, a clear distinction is needed between the orientation used 1) for CT
data acquisition 2) for treatment planning and 3) for treatment delivery. It is expected that theorientation would remain the same for all steps and that the consistency would be guaranteedby the use of DICOM standardization. However, there are many possible combinations andspecial circumstances where it might be necessary to setup the patient with a given orientation
but to “declare” another orientation for any of these 3 steps. There is then a potential for errorthat could lead to severe accidental exposures.
3.3.2.2.
(54)

Tissue density values from CT data
The direct connection of CT numbers with tissue density required for subsequent

dose calculation is an important and very useful feature but also brings new risks. If artifacts
are present or if a contrast medium is being used, it might be incorrectly interpreted as apatient organ density. Magnetic resonance imaging (MRI) can produce image distortion.
Combining several imaging modalities brings an additional risk of misregistration 2, resulting
in a significant error in the location of either the target or a critical structure.
3.3.2.3.
(55)

Consistency of coordinates and beam characteristics
Virtual simulation requires that the coordinate origin and conventions are correctand consistent between image data acquisition, virtual simulation, dose planning, and2

Registration is the process of linking image data from different studies to a single coordinate system, typicallythat of the treatment planning CT.

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treatment delivery. There is another potential error arising from using the beam characteristics
defined at the time of simulation (sometimes called “set-up fields”) directly for treatment
planning or delivery, without checking that all properties that could influence monitor units(MU) or dose calculation are correct. For example, accessories which are not required for thesimulation process (e.g. trays, wedge filters) should be added as necessary when using thesebeams for treatment. It could also happen that simulated beams are not used for treatment or
are modified, but that they are mistakenly kept by the system and then used.
3.3.2.4.
(56)

Significant exposure from imaging
The growing importance of imaging in the treatment planning process and the

reduction of CT image acquisition times are likely to increase the number of pre-treatment or
post-treatment x-ray examinations and therefore the patient exposure to diagnostic quality xrays. One example is the use of 4D CT acquisition where each single slice is replaced
potentially by 10 slices acquired at different phases of the respiratory cycle. This additional
exposure for imaging which, until recently was considered negligible as compared to thetherapeutic exposure, is becoming significant and needs to be taken into account and assessed
prior to undertaking a CT based preparation (Murphy et al. 2007).
3.3.3. Image segmentation and structure delineation(57)

Image segmentation is an image processing method to enhance and distinguish anobject or its boundaries (lines, curves) from the rest of the image. In radiation therapy, this is
typically used to more clearly locate and delineate organs or structures in images. This phaseof treatment preparation is performed using either the tools of the treatment planning system(TPS) or with software provided specifically for virtual simulation. There are a number ofpotential errors, which depend on the software tools that are available and on the requirements
for the subsequent phase of planning, for example, the system capabilities for automatic
external or internal structure extraction or for 3D expansions. In most cases, improper tools or
improper use of them will result in a loss of geometric accuracy with a possible underdosage
at the periphery of the target volume and overdosage of normal tissue without a change of thedose at the reference point. However, dose planning algorithms may inherently have someconstraints on how the anatomical model should be prepared (number or spacing of slices,
contour regularity, absence of intersections or overlapping regions, CT number allocations,etc.) that may not be explicitly identified by embedded warnings or interlocks. If such

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warning and interlocks are not present and if the user does not understand the limitations of
the anatomical modeling, severe errors may appear in the subsequent handling of this data forbeam set-up or dose calculation. For example, some algorithms will not reconstruct a
continuous patient surface from a limited number of CT slices. The calculated dose
distribution and number of monitor units would be then affected by some error, especially if
the beam axis lies between distant slices or if a non-coplanar beam is used across a top sectionthat is considered as “empty”. Another example is if the user forgets to assign a density to aslice defined from a contour.
3.3.4. Beam definition and optimization of the dose distribution(58)

This step consists essentially of calculating the dose distribution resulting fromthe beam set-up proposed for treatment. This step is carried out using a computerized
treatment planning system (TPS). However it is clear that TPSs are no longer restricted to thecomputation of dose distributions. Instead, they can be considered essential pieces of theprocess of transforming the radiation oncologist’s prescription (dose, volume, time pattern)
into an optimized plan, ready to be transferred to the treatment machine. Although some past
accidental exposures have been caused directly or indirectly by the use of TPSs (IAEA,
2001), it is generally recognized that the main source of error comes from a poor
understanding of some of the TPS functionalities combined with a lack of redundancy in thedetermination of the number of monitor units (double check with independent calculation
and/or in vivo dosimetry). Before using any TPS for real treatments, a time consuming
commissioning phase is indispensable and the incorrect input of basic parameters may lead tosystematic errors. In addition, errors may also occur in the TPS daily use.
(59)

A typical list of the main tasks performed when using a TPS is given in Table 3.1.

With each task, a risk index has been associated. The risk index combines an estimation of theprobability of occurrence and severity (adapted from the French Society of Medical Physics(SFPM)) of an error. The purpose of this table is to help build a safety system with emphasison the most hazardous steps such as the beam configuration, the use of wedge filters (or otherbeam modifiers), the management of beam weights and the computation of monitor units(MU). The risk indices are defined as follows :1 : geometrical or dosimetric deviation from the « correct » plan without expectedsignificant clinical consequences (low severity)
2 : low probability, high severity

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3 : high probability, high severity
Table 3.1. List of tasks performed when using a treatment planning system (adapted from SFPM,
2006).
Task
Preparation of the beam datalibrary (parameterization)
Patient anatomical data acquisitionand data transfer to TPS

Risk index
3

2

1

model
Definition of shapes and densities
for inhomogeneous regions
Target and critical organs
delineation
Target volume expansion
Choosing treatment machine,modality and energy

Critical step, particularly regarding reference dose
rate and output factor
Main risk is related to patient orientation
management (see section 4)
It has direct influence on the tissue thickness used

Delineating the external contour
and building the patient anatomicalComment

for monitor unit (MU) calculations; there is a
specific risk from top and bottom slicecharacteristics, especially for non coplanar beams

1

1

1

2

It has direct influence on MU calculations
It has direct influence on beam set-up and dosevolume histograms
It has direct influence on beam set-up and dosevolume histograms
In this step there is a risk if using obsolete data andof consistency with actual equipment characteristics
In this step, critical issues are: the distinction
between SSD and isocentric techniques: the meaning

Beam set-up definition

2of the displayed coordinates (e.g. if SSD is different
from SAD) and the consistency of collimator andtable rotations scales
No serious risk if there are embedded safety features

Defining field shape1

preventing wrong input (cf. Panama accident, IAEA
2001)

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Risk index

Adding beam modifying devices(shielding blocks tray, wedge

Comment
Critical here is the awareness of the presence and3filters, compensators…)

nature of the modifiers since they have a strong
influence on MUs
Critical in this step is the avoidance that the beam

Choosing beam weighting points

3

waiting points are not lying in a region where dose islow (e.g. under a block) or where the dose gradient ishigh (e.g. field edge)

Defining (total or fractional) beam
weighting (contribution)
Dose distribution calculation anddisplay
Dose volume histograms
calculation and display

Decision on final approved
treatment plan2

1

It has direct influence on MU calculations;sometimes difficult to understand the exact meaning
Critical issue is awareness of calculation options
Critical issue is awareness of calculation options and1volume definitions (are structures completely
enclosed into sampling region?)
Critical issue when several studies have been2

performed, is assurance that the approved plan will
be actually used for treatment
Could have been achieved prior to the final plan

Monitor unit calculation

3

approval or could be performed on a separate
system; critical step is verification of all relevant
data

Data transfer from TPS totreatment machine

(60)

3

Another critical step where the verification of all
relevant data

Modern TPSs are very complex and offer a full range of functionalities with many

possible pathways. It is, therefore important to follow formal acceptance testing procedures tocheck all possibilities. On the other hand, all systems are inherently likely to fail when used
under special circumstances. Most of the failures would result in a system crash with no otherconsequences than a loss of data and time. Only in very peculiar circumstances would some
failures influence the treatment outcome. Trying to control these circumstances by instituting
preventive actions (e.g. through redundancy) is precisely the aim of a systematic safety

32

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assessment approach as described in Chapter 5. Such risk can be reduced by providing thesystem with a fluent user interface and sufficient warnings and interlocks. However the mostimportant risk is associated with human error, related to inappropriate use of somefunctionalities because of insufficient training or poor understanding of some aspects of the
TPS. Such errors have direct implications for treatment quality and safety. Very severe
accidental exposures of this type have already occurred in conventional radiation therapy.
3.4. Treatment delivery•

A consistent coordinate system is to be used in all steps from virtual simulation
through treatment planning to treatment delivery, in order to avoid errors.
Treatment delivery may require a manual shift of the patient couch to correctly
place the patient with respect to the treatment beams. With manual shifting, there isa risk for error if there is no systematic image verification, and if coordinate systems
are inconsistent. This risk is even higher if the delivery system does not allow thevisualization of the actual irradiation field, for instance in tomotherapy.

•

Working with geometrical coordinates may lead to a tendency to focus too much on
coordinates and to lose awareness of what is actually being treated (for example
treating the correct side in the case of a lateral tumor)•

Systems for automatic matching of image structures against a reference image are a
substantial improvement in patient positioning and for inter and intrafraction
motion correction. On the other hand, errors and mistakes when taking or using thereference image may be difficult to detect and may affect the whole treatment
course.

•The radiation dose from portal imaging may be significant and can lead to a
deviation from the prescribed dose, if not taken into account and integrated into
treatment planning and delivery.

•

Small fields pose additional challenges in their measurement and calibration andmay, therefore, require a review of training and the manufacturer’s system
explanatory notes and warnings.

•

With dynamic wedges the output varies with the moving jaw position and thisvariation depends very much on the manufacturer’s design. Without proper

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information and alerts from the manufacturer there is a significant risk of error
during the calibration (or commissioning) procedure.
(61)

The actual delivery of the treatment can start as soon as the plan has beentransferred from the TPS to the treatment machine. The transfer process is part of patient data
management and is dealt with in Section 3.5. In order to guarantee that the treatment is given
accurately at the proper anatomical location, the patient and beam set up has to be kept
consistent according to the plan prepared during virtual simulation and/or during doseplanning.
3.4.1. Verification of patient and beam setup(62)

Verification of the patient position relative to the beams requires that the patient isproperly immobilized. The risks related to fixation or immobilizing devices have been alreadyaddressed in section 3.3.1 and will not be further discussed here.
3.4.1.1.
(63)

Coordinates and external marks and references
Historically, patient setup relied on skin marks, drawn during simulation at thecenter and/or at the edges of the treatment fields. In modern systems, adjustment of the patient
position with respect to the linear accelerator coordinate system quite often requires atranslation (shift) of the table coordinates with respect to a patient reference coordinate
system.
(64)

The patient reference coordinate system is determined by the alignment of radio-

opaque and/or tattooed skin markers to the light projection from the wall mounted lasers,
while the required shift is determined during virtual simulation or from dose planning. The
shifting method is necessary unless the patient is firmly fixed to the table with an indexing
system allowing the use of absolute coordinates.
(65)

Relative shifting is carried out preferably with table scales, or by measuring with aruler. Although some systems allow the table coordinate origin to be reset to the origin of thepatient reference coordinate system, in most systems the shift needs to be accounted for by anaddition or subtraction from the actual absolute table position. It is then possible to commit anerror and use the wrong value or wrong direction for patient positioning. This risk is evenhigher if the system does not allow the visualization of the actual irradiation field, for instancein tomotherapy.

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THERAPY TECHNOLOGIE(66)

As a consequence of this ‘new’ approach of working with geometricalcoordinates, technologists may tend to focus on the coordinates and to lose very basic
awareness of what they are treating (for example treating the correct side in the case of alateral tumor). There is also a tendency for them to rely quite heavily on the record and verify
system which would not necessarily catch such errors (i.e. table coordinates without humansupervision or too loose tolerances).
3.4.1.2.
(67)

Position verification against patient internal structures
X-ray imaging is required for the verification of patient position by visualizing

internal structures. This was traditionally achieved with radiographic portal films, which havebeen replaced more recently by electronic portal imaging devices (EPID). Typically, theverification takes place on the first treatment day or one day before. It may be repeatedseveral times at the very beginning of the treatment course and once every week thereafter.
With higher demand on the conformality of the dose distribution and with margin reduction,an accurate patient setup is required and this can only be guaranteed if image based
verification is repeated more often. Ideally a daily check could be recommended but then the
additional radiation dose given to the patient may be no longer negligible. This is particularly
the case when low sensitivity systems are being used (typically liquid ionization chamberbased EPID) but, even with the development of relatively high sensitivity amorphous silicondetectors, if this dose contribution is ignored, it could easily result in an over dosage
equivalent to one fraction for a full treatment course. Several solutions may be applied to deal
with this problem: compensation at each fraction for the additional verification monitor units,
overall compensation at the end of the treatment or adjustment of the prescription andintegration of the image dose contribution into the patients’ treatment plans.
(68)

A further difficulty arises because the region which has to be imaged forverification purposes is not strictly limited to the target volume and could possibly includesensitive structures. This is well known from the double exposure techniques which were usedin conjunction with radiographic portal films. It also happens when standard beamorientations (typically AP and lateral) are used to verify the position of a patient treated withoblique incidence (coplanar or non coplanar beams) and for the more advanced techniques ofimage guided radiation therapy (IGRT), where the recommended protocol could be a dailyacquisition of cone beam computerized tomography (CBCT), serial tomography

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THERAPY TECHNOLOGIES(tomotherapy) or standard in-room x-ray image pairs (proton therapy, cyberknife…). In allcases the resulting dose should be assessed and taken into account.
3.4.1.3.
(69)

Position correction
Correction of patient position is applied according to a strategy and procedure

under the overall responsibility of the radiation oncologist, even if the task is delegated totechnologists or it is done under the supervision of medical physicists. Formerly, theadjustment of the patient position was performed by technologists essentially from oral or
written instructions given by radiation oncologists from what they saw on portal images (e.g.
“move the patient 0.5 cm in the cranio-caudal direction and 1 cm to the left”). This approachis being replaced by image or structure matching approaches where the patient shift aims at
superimposing the current on-treatment image on a reference image (e.g. DRR) obtained fromvirtual simulation or from the TPS. The correctness of the shift is validated (or not) bycomparing with the image in the corrected position with the reference (implying additionalradiation dose). Apart from the additional exposure as discussed above, there is a risk ofrelying too much on the reference image which may be wrong due to errors made during
virtual simulation or at the TPS. Such errors, if they occur, would be difficult to detect andwould likely be there from the beginning up to the end of the treatment course.
3.4.2. Intensity Modulated Radiation Therapy and other advanced dynamic techniques(70)

In the last decade there has been an impressive development of intensity

modulated radiation therapy (IMRT) made possible by the advent of computer assisted
inverse planning and technical solutions to accurately control the field shape of multileaf
collimated (MLC) fields as a function of delivered monitor units. More recently tomotherapy
and the cyberknife offered additional degrees of freedom, the former allowing 360° rotationof the x-ray source around the patient and continuous table motion, the latter providing
optimized directions of hundreds of minibeams emerging from an accelerator mounted on a
robotic arm. Other new techniques include intensity modulated arc therapy (IMAT) andhadrontherapy employing photons and heavy charged particles respectively.
(71)

In spite of the inevitable pressure to achieve improvements by using newtechnology and of the wish of the radiation oncology community to make the benefits
available to patients as soon as possible, the clinical application of these advanced techniques
has been, in general, kept reasonably well under control. Specific quality control procedures
have either been developed and published by advanced groups in charge of clinical tests

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before commercialization or provided by the manufacturers as part of the purchased“package”. An example of such a procedure is the quite common practice of pre-treatment
plan validation on a phantom (sometimes called “hybrid plan”) before any IMRT treatment3.
Spreading these new techniques over a larger number of centers or involving a larger numberof patients needs to be achieved progressively, accompanied by adequate quality control
procedures (IAEA, 2008). Several factors may increase the risk of accidental exposure, if
appropriate safety barriers are not implemented:- The multiplication of the number of parameters to be monitored during treatment makes
them more difficult to keep under control.
- The mechanical aspects of the robotic approach, which is being used to control the
spatial position of the accelerator or of the patient, is a potential source of danger(collision or failure of the control systems).
- The development of “segmented” irradiation techniques, in which coverage of large
irradiation volumes is achieved by the superposition of many elementary dose
distributions puts stricter requirements on instantaneous dose rates. Such techniques
could be static (step and shoot mode) or dynamic (sliding window, tomotherapy,
multiconvergent robot driven beams…). One important safety issue is the ability tostop the beam fast enough to avoid significant overdosage in the case of failure of one
component. This is particularly relevant for dynamic techniques (including scanningbeam approaches) where severe overdosage can occur in case of failure of themechanical or electronic system used to scan the beam over the entire treatment
volume.
(72)

However, as in other branches of complex activities with potential for major

accidents (aeronautical and nuclear industries) the technology is mature. Provided that theequipment is developed with proper consideration of safety issues according to industrial
standards (IEC, 1997, 1998, 2000, 2005) and with reinforcement of safety interlocks

3

Such a practice is representative of what should be done when starting with a new complex procedure where
there is concern for some unexpected event with possible consequences for safety or accuracy. However, aftersome time and experience, these procedures could be revised and simplified as staff become more confident
with equipment stability and safety. This is slowly being introduced in centers with the larger experience in
IMRT which tend to abandon systematic pre-treatment verification and replace it either by reinforced specificquality control of the equipment or by systematic in vivo exit dose measurements with EPID.

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compared to more traditional treatments (e.g. redundancy), there should not be significant
additional risk directly related to the implementation of such advanced technology. The
following issues deserve separate discussion:
3.4.2.1.
(73)

Exposure outside the target
The segmental nature of most IMRT techniques implies not only an increase of

the instantaneous dose rate but also an increase of the total number of MU required for agiven dose to the target. Therefore, the dose at distance from the irradiated volume due tocollimator leakage or head scatter, although generally negligible in conventional radiation
therapy, might become significant for IMRT techniques. The neutron dose at a distance fromthe target volumes treated with hadron beams and passive techniques (scattering foils andmechanical modulator) is also relevant to protection and safety. The quantification of thesecontributions and their impact on the risk of second cancers is still debated and is the subjectof a separate ICRP/ICRU report (in preparation).
3.4.2.2.
(74)

Complex dose measurements in combined small beams
Dose measurements are more difficult to perform when small static or dynamic

beams are combined to create the required dose distribution (IMRT, multibeam radiosurgery
tomotherapy…). The choice of the appropriate detector (size, energy response, calibration…)
and the experimental setup for beam calibration are of utmost importance. Although the users
are clearly responsible for properly commissioning such systems, which should be preferably
done with the help of experienced colleagues and/or in the framework of structured networks
or user groups, manufacturers should alert users to the configuration and complexity of suchdevices and their implication on dose measurements.
3.4.2.3.
(75)

Software control of accelerator output
The output of computer-controlled accelerators are much harder to predict using

physics rules since the response of the controlling or measuring devices can be corrected
electronically or by software look-up tables more or less easily accessible to users. For
example, most often the monitor response is the result of a computation which can include, ornot, corrections related to the collimator opening or presence of wedges. As an illustration, theoutput of the one particular accelerator brand in the presence of “enhanced wedges” varies

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rapidly as a function of the position of the fixed jaw (and not as a function of the equivalent
field size) whereas, for another type of accelerator, virtual wedges (which are based on thesame general principle) the output is practically independent of the jaw position. The reasonsfor these differences are rather complex and may be explained by the detailed difference incontrolling electronically the dose rate or cumulative dose as a function of the moving jaw
position (Leavitt et al. 1997, Liu et al. 1998, van Santvoort 1998, Faddegon and Garde 2006).
Without proper information and alerts from the manufacturer there is a significant risk oferror during the calibration (or commissioning) procedure.
3.5. Patient data management•

Communication among different components, including those from different
manufacturers, requires standardized transmission control protocols and the
DICOM format. However, there may be still a number of pitfalls. Examples arepotential misinterpretation of certain data by a device, such as of data entered in an
optional DICOM field, or errors in data conversion to match data used by different
devices from different manufacturers.

•

Record and verify systems substantially increase protection against human errors indaily treatments, but they leave scope for other risks. Reliance on a system which isassumed to be error free may lead to overconfidence and to relaxing attention;
errors may arise in special treatments, or modification of treatments, requiring
manual entry of additional data or when transferring a patient from one machine toanother. The manual data selection for conventional daily treatments, which was
subject largely to random errors, is now replaced by single data entry to the system,
which, if done incorrectly, may turn a random error into a systematic one affecting
the whole treatment course.

•Electronic chart functionalities facilitate moving towards a “paperless” department,
but without careful backup planning, data availability may be compromised. The
reference dose, meant to give a warning when that dose, or number of fractions, isreached, has the potential for failure under certain conditions, for example, in thecase of a machine fault, when the treatment session is moved to another machine or
to a non-working day.

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THERAPY TECHNOLOGIES•Image handling systems with automatic image numbering and automatic imagetransfer add reliability, but when image related information needs to be enteredmanually, errors can be electronically propagated through the whole chain.

3.5.1. Description of existing patient data management systems(76)

The use of computerized data management systems as the backbone of theorganization of a radiation therapy department makes it necessary to have a clear perceptionof the general workflow and to interface the different components in such a way that dataexchanges are safe and reliable. There were several steps in the historical development ofpatient data management systems :3.5.1.1.
(77)

Record and verify systems
Earlier systems were the so called “Record and Verify Systems” (R&V) whichwere interfaced with treatment machines. They used a database containing, for each patient,
the prescribed machine parameters (i.e. gantry angle, field size, number of MU/beam, etc.)
obtained either from the TPS or from the simulator. For each fraction and each beam thesevalues were automatically checked against the actual machine parameters that were set
manually. In the case of a deviation larger that some predefined tolerance, a warning was
displayed and an interlock prevented the treatment from starting. Additionally, for eachfraction, the main machine parameters actually used for the treatment were recorded andcould then be reviewed.
(78)

The functionalities of record and verify systems are still of major importance forthe radiation therapy process. One of them is the verification of the machine setup (the “V” of
R&V) which improves safety but also introduces some risks as discussed in Section 3.5.2.
The second of these is the possibility to automatically fill-in a document which traces thedetails of the patient treatment in the form of an “electronic patient chart” (the “R” of R&V),
as discussed in Section 3.5.3.
3.5.1.2.
(79)

Radiation therapy information systems
Modern systems are no longer strictly R&V systems but they have expanded into“radiation therapy information systems” (RTIS) that integrate more or less effectively manydifferent components of the patient workflow, such as management of administrative data

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THERAPY TECHNOLOGIE

such as billing, management of internal or external resources (people, equipment, rooms),
scheduling and creation and updating of the patient treatment charts, etc. These softwaresystems might also include image management with picture archiving and communications
system (PACS) functionalities. They might be huge and difficult to understand properly(Fraass 2008). They are meant to interface with other in-house existing databases and requireassistance from computer specialists.
3.5.1.3.
(80)

Direct control of machine parameters
In addition to RTIS, with the development of IMRT, there are new requirements

for direct software control of the machine parameters in order to automatically sequence thebeams and drive the components used for beam modulation (MLC, cumulative MU, gantry
rotation, etc.). Then the system is no longer limited to “verifying” that the machine parameters
are correct but it directly assumes control. Therefore other means should be found toguarantee the consistency between prescription and delivery.
3.5.1.4.
(81)

Communication among different components
The communication between the different components of an integrated RTIS isgenerally achieved by a standardized approach such as “transmission control protocol /internet protocol” (TCP/IP) network, digital imaging and communication in medicine(DICOM) format and DICOM RT based exchanges. This provides flexibility and allows
integration of equipment from different companies. However the variety and the complexityof the available solutions can lead to many potential pitfalls, essentially due to theintroduction of data in the optional fields of the DICOM format or to a particular use of someplanning or treatment data by one manufacturer’s device that can be misinterpreted by theothers.
3.5.2. Machine setup verification functionalities of “record and verify” systems(82)

The verification functions of “record and verify” (R&V) systems were designed toincrease the reliability and safety of the radiation therapy process, against human error indaily treatment delivery. These features cannot be circumvented when complex techniques areused because it entails dealing with a very large number of parameters (e.g. for IMRT) which
could not be set manually. It has been demonstrated that R&V systems are effective in
detecting random errors which are operator dependent (e.g. checking the presence of wedge

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THERAPY TECHNOLOGIES

filters or the number of monitor units) (ref.). However they are also responsible for new typesof errors (Fraass et al. 1998a, Patton et al. 2003), for the following reasons:•

The daily use of an R&V system has an impact on the state of mind of technologists
who know that there is an automatic safety system working in the background, andwith this state of mind, they inevitably tend to relax their attention as compared to amanual system which would be under their control. A typical example is theapplication of the treatment parameters of a wrong patient (ref.) by simply clicking
on the wrong line as part of a process which is becoming highly repetitive.

•

The data of an R&V system are normally exported from the TPS through a networkthat should provide an error free solution provided that the whole system is properly
commissioned. However, the transfer can be incomplete for some treatments whichthen requires additional manual data input. Such manual input is prone to error, butwith the false confidence of working with an “error-free” system.

•

As for TPS, the number of possible pathways of so many functionalities is verylarge and occasional errors may occur in special circumstances such as purposely
modifying a plan for a treatment that has already started or transferring a patientfrom one machine to another.

•

R&V systems are strongly interdependent on the other components of the radiation
therapy network. Some manufacturers are even integrating the TPS functionalities
as one module of the full radiation therapy information system. Thus, although anumber of safety interlocks are normally included, it has become very difficult toclearly understand the possible consequences of any action performed on the patientelectronic chart leading to an increased risk of misinterpretation and error.

•

The presence of an R&V system is likely to change into systematic (and therefore
much more severe) many of the errors that would have been random for a manuallybased treatment setup (Fraass et al. 1998a, Huang et al. 2005). As an example, anincorrect field size setting or the unintended omission of a wedge that could occur
on one day in manually based treatment setup, as the result of a human error, would
occur systematically every day with an R&V system, if the that error is made whenintroducing the data into the R&V system. This consideration should not beinterpreted as a discouragement to use R&V systems (which have also the

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THERAPY TECHNOLOGIE

advantage of giving access to statistics on error rate) but as a warning about thepotential shift from one type of error to another one (Goldwein et al. 2003)
3.5.3. Electronic chart functionalities of “record and verify” systems(83)

Automatic recording of treatment parameters for each fraction offers thepossibility to dispense with the traditional paper sheet where technologists manually record
the basic information related to each fraction (e.g. the number of monitor units), for eachpatient and each beam. This step leads to evolving into a “paperless” department.
(84)

Such a change is difficult and requires careful analysis for the following reasons :•The availability and reliability of computer systems and data have to be guaranteed.
Backup solutions require careful thought in advance to cover any failure situation.

•

The patient chart is the traditional location where one can find handwritten
information related to the treatment prescription, the treatment execution andinstructions for any change occurring during the treatment course. In principle, thereplacement by a computerized system has the advantage of forcing all categories ofstaff to fill-in the needed information. It is then well adapted to the standard
processes as defined in existing protocols. However, for any special situations (e.g.
a modification after the treatment has actually started) it could be difficult to find aproper replacement solution for smooth and safe communication between
professionals, depending on the tools available in the computerized system and of
the local rules for describing unexpected events that could happen during atreatment course.

•

Special attention has to be paid to the follow-up from the delivery of each fraction.
Most R&V systems offer the possibility of accumulating the dose at one (or several)
reference point(s) as the treatment proceeds and of giving a warning if the dose (ornumber of fractions) exceeds the prescribed value. There are several possibilities for
misreporting in the electronic chart that should be carefully investigated (e.g.
machine failure, treating the patient on another machine or a non-working day) andhence there is the risk of either repeating a session that has not been recorded or
stopping before the end of the course, if technologists rely too heavily on beingalerted automatically by the system.

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THERAPY TECHNOLOGIES(85)

It is difficult to review the numerous possible pitfalls related to the introduction ofan electronic chart recording system in a radiation therapy department. It is, therefore,
important to develop thorough procedures and to plan a commissioning phase and a “probing
period” before such a system may be used safely. Proactive systematic approaches to find out
what can go wrong, are described in Chapter 5, and they can be used as tools to develop suchprocedures and commissioning plans.
3.5.4. Image handling(86)

Images are becoming an essential component of the patient record and somesystems incorporate them directly into the radiation therapy information system. These
images may be :•pictures taken to confirm the patient identity or to help in patient set-up•

diagnostic images•

images used in for patient anatomical reconstruction•reference images, either acquired directly at a simulator or digitally reconstructed(the so called “digitally reconstructed radiographs” or DRR)•

portal images used to confirm the field shape and its position with respect toanatomical structures•verification images (kV or MV images of orthogonal verification beams, CT or cone
beam CT images) acquired in the treatment room to confirm patient position withrespect to the machine isocenter(87)

In all cases careful image identification is required and there is a need to know

without ambiguity not only to which patient the images belong but also to have access to
information such as the date and time, the device used for imaging, the geometrical imaging
characteristics, the doctor or technologist who performed the image acquisition, the related
beam and session number whenever relevant. Image orientation with respect to the patient andto the beam should also be known precisely. DICOM standardization helps to ensure that suchinformation is attached to digital images and automatic image transfer reduces the risk oferror. However, in many cases, it might be necessary to manually complete image related
information and this could cause the propagation of an erroneous identification throughout thewhole chain.

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THERAPY TECHNOLOGIE(88)

The most frequent and most severe cause of error may be related to the reference

images used to check or adjust the beam position with respect to the patient. If, in the
database, they are manually attached to the relevant beam there is a possibility of using as
reference either another beam of the same patient or a similar beam from another patient orfrom another plan than the approved one. The consequence could be a systematic geometricmismatch that would remain undetected during the whole treatment course. In all cases,
images play an important role in the safety and precision of treatment delivery but errors
could occur leading to significant consequences for treatment outcome.
3.6. References
Das, I.J., Chee-Wai Cheng, C.W., Kashmiri L. Chopra, K.L., Raj K. Mitra, R.K., Shiv P.
Srivastava, S.P., Eli Glatstein, E., (2008). Intensity-Modulated Radiation Therapy Dose,
Prescription, Recording, and Delivery: Patterns of, Variability Among Institutions andTreatment Planning Systems. J Natl Cancer Inst 2008;100: 300 – 307.
Faddegon, B. A., Garde, E. (2006), 'A pulse-rate dependence of dose per monitor unit and its
significant effect on wedged-shaped fields delivered with variable dose rate and a moving
jaw.', Med Phys 33(8), 3063--3065.
Fraass, B.; Doppke, K.; Hunt, M.; Kutcher, G.; Starkschall, G.; Stern, R. & Dyke, J. V.
(1998), 'American Association of Physicists in Medicine Radiation Therapy Committee
Task Group 53: quality assurance for clinical radiotherapy treatment planning.', Med Phys
25(10), 1773--1829.
Fraass, B. A.; Lash, K. L.; Matrone, G. M.; Volkman, S. K.; McShan, D. L.; Kessler, M. L. &Lichter, A. S. (1998a), 'The impact of treatment complexity and computer-control delivery
technology on treatment delivery errors.', Int J Radiat Oncol Biol Phys 42(3), 651--659.
Fraass, B. A. (2008), 'QA issues for computer-controlled treatment delivery: this is not yourold R/V system any more!', Int J Radiat Oncol Biol Phys 71(1 Suppl), S98--S102.
Goldwein, J. W.; Podmaniczky, K. C. & Macklis, R. M. (2003), 'Radiotherapeutic errors andcomputerized record/verify systems.', Int J Radiat Oncol Biol Phys 57(5), 1509; author
reply 1509--1509; author reply 1510.

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THERAPY TECHNOLOGIES

Huang, G.; Medlam, G.; Lee, J.; Billingsley, S.; Bissonnette, J.; Ringash, J.; Kane, G. &Hodgson, D. C. (2005), 'Error in the delivery of radiation therapy: results of a quality
assurance review.', Int J Radiat Oncol Biol Phys 61(5), 1590--1595.
IAEA International Atomic Energy Agency, 2001. Investigation of an Accidental Exposure ofRadiotherapy Patients in Panamá. International Atomic Energy Agency. IAEA, Vienna.
IAEA International Atomic Energy Agency (2008). Design and Implementation of aRadiotherapy Programme: Clinical, Medical Physics, Radiation Protection and Safety
Aspects. (IAEA, Vienna).
IAEA International Atomic Energy Agency (2004). Commissioning and Quality Assurance ofComputerized Planning Systems for Radiation Therapy of Cancer. IAEA TRS-430 (IAEA,
Vienna).
ICRU, 1993. Prescribing, Recording and Reporting Photon Beam Therapy, Report 50,
International Commission On Radiation Units And Measurements, Bethesda, MD
ICRU 1999. Prescribing, Recording and Reporting Photon Beam Therapy (Supplement to
ICRU Report 50), International Commission On Radiation Units And Measurements,
Report 62, ICRU, Bethesda, MD.
IEC, 1997. General Requirements for Safety. 4. Collateral Standard: Programmable Electrical
Medical Systems. Publication. International Electrotechnical Commission IEC 60601-1-4,
Geneva (1997).
IEC, 1998. Part 2: Particular Requirements For the Safety of Electron Accelerators in the
Range Of 1 To 50 Mev, International Electrotechnical Commission

IEC-60601-2-1,

Geneva (1998).
IEC, 2000. Medical Electrical Equipment: Requirements for the Safety of Treatment Planning
Systems. International Electrotechnical Commission,Publication IEC-62C/62083, Geneva
IEC, 2005. Medical Electrical Equipment: Safety of Radiotherapy Record and Verify
Systems. International Electrotechnical Commission,Publication IEC-62C/62274, Geneva
Leavitt, D. D.; Huntzinger, C. & Etmektzoglou, T. (1997), 'Dynamic collimator and dose rate
control: enabling technology for enhanced dynamic wedge.', Med Dosim 22(3), 167--170.
Liu, C.; Li, Z. & Palta, J. R. (1998), 'Characterizing output for the Varian enhanced dynamic
wedge field.', Med Phys 25(1), 64—7

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Murphy, M. J.; Balter, J.; Balter, S.; BenComo, J. A.; Das, I. J.; Jiang, S. B.; Ma, C. M.;
Olivera, G. H.; Rodebaugh, R. F.; Ruchala, K. J.; Shirato, H. & Yin, F. (2007), 'The
management of imaging dose during image-guided radiotherapy: report of the AAPM
Task Group 75.', Med Phys 34(10), 4041--4063.
Patton, G. A.; Gaffney, D. K. & Moeller, J. H. (2003), 'Facilitation of radiotherapeutic error
by computerized record and verify systems.', Int J Radiat Oncol Biol Phys 56(1), 50—57
Rosenwald, J. C. (2002), '[Safety in radiotherapy: Control of software and informatics
systems]', Cancer Radiother 6 Suppl 1, 180s--189s (in French).
SFPM Recommandations pour la mise en service et l’utilisation d’un système de planification
de traitement en radiothérapie (TPS), rapport de la Société Française de Physique
Médicale (in preparation)
van Santvoort, J. (1998), 'Dosimetric evaluation of the Siemens Virtual Wedge.', Phys Med
Biol 43(9), 2651--2663.

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4. REPORTED ACCIDENTAL EXPOSURES WITH NEW TECHNOLOGIES• A number of reports of accidental exposures involving new technologies has
become available in recent years and these provide valuable lessons forprevention. Analyses of lessons learnt confirm and substantiate most of the safetyissues and potential for errors identified Chapter 3.
(89)This chapter contains descriptions of representative cases reported in theliterature, to the Radiation Oncology Safety Information System (ROSIS) or to the Event
Notification Reports of the United States Nuclear Regulatory Commission. Each case relates
to some specific new radiation therapy technology or procedure. For each case, a summary of
the case history is presented, followed by a discussion on the lessons to learn from the event.
Short descriptions of more events are included in the appendix.
4.1. Events related to beam output and calibration
4.1.1.

Calibration problems of small-fields

Case 1: Inappropriate detector size used when commissioning micro-multileaf collimators(ASN, 2007, Derreumaux, 2008).
(90)

A characteristic of micro-multileaf collimators is the ability to form very small

irradiation fields with high precision. These fields can be used when irradiating small targets
in, for example, the brain, such as in radiosurgery applications. When commissioning atreatment unit equipped with a micro-multileaf collimator, beam data should be collected byusing dosimeters with an appropriate detector size considering the potentially small size of theirradiation fields.
(91)

In April of 2006, a hospital physicist commissioned a new stereotactic unit

capable of operating with micro-multileaf collimators (3 mm leaf-width at isocenter) orconical standard collimators. With this unit it is possible to shape clinically usable fields
down to the very small field size of 6 mm x 6 mm. When collecting beam data for thetreatment planning system, it is necessary to measure the beam dose characteristics down tothis field size. The beam data is subsequently used for treatment planning purposes. Data
collected for the micro-multileaf collimator is handled separately from data collected forstandard collimators.

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When measuring absorbed doses and collecting beam data (scatter factors) forvery small beams formed by micro-multileaf collimators, the physicist at the hospital used a
Farmer 0.6 cm3 ionisation chamber, which is too large for this type of measurements.
Consequently, the dose measurements were incorrect for all small micro-multileaf collimatedfields, whereby the calibration files for all micro-multileaf collimated fields were wrong. This
caused the wrong absorbed dose to be administered when these fields were used with amaximum overdose of ~200%. Patients treated with conical standard collimators were notaffected.
(93)

The anomaly in the calibration files from the hospital were discovered by thevendor some time later, when they were reviewing the calibration files collected from
European centers. They informed the hospital in April of 2007. At this stage, 172 patients had
been treated stereotactically on the unit. 145 of them had been treated using micro-multileaf
collimators and had thus been affected by the erroneous measurement. In most cases, thedosimetric impact was evaluated to have been small. However, patients were identified forwhom tolerance doses in normal tissues and organs were exceeded.
Discussion and lessons:(94)

The use of micro-multileaf collimators poses new challenges to physics

knowledge and expertise. It, therefore, requires revisiting staff qualifications, i.e., verificationthat physicists in the department have a thorough understanding of the new technology, itsfeatures and the measurements to be performed. In particular, full knowledge is required of
the small fields involved with micro-multileaf collimators, of the conditions they impose on
detector size, and of the physical effects of a partial irradiation of a detector that is larger thanthe beam cross-section.
(95)

After revisiting the training issues a thorough preparation for commissioning

would have been necessary. This includes the preparation of procedures for the specificmeasurements with micro-multileaf collimators, or a conscious decision on the adoption of
relevant procedures from recognized protocols(96)

Finally, independent checks of the measurements and calculations, andclarification of any discrepancy would have been necessary before the radiation therapy
equipment is clinically used. An independent check would have been further enhanced, forexample by inviting physicists from another hospital to check measurements and calculations

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using their own equipment and calculation methods. With these measures in place, such an
accidental exposure would be very unlike to occur.
4.1.2.

Intra-operative radiation therapy beam calibration issues

Case 2: Intra-operative calibration error from a wrong calibration file (ROSIS, 2008)(97)

New intra-operative radiation therapy (IORT) equipment was delivered to a clinic.

The clinic received no information on how the absorbed dose at specific distances from theintra-operative applicators was measured by the manufacturer, including measurement
geometry, and thus how the pre-installed calibration files containing the information required
for the calculation of treatment times were devised.
(98)

A phantom was created locally to measure the absorbed dose on-site in the clinic

at the commissioning of the IORT equipment. Each single applicator had a specificcalibration file provided by the manufacturer. During commissioning, it was noted that thetwo applicators with a diameter of 4 cm were equal in all geometrical aspects but differed by
20% in measured output in relation to each other when the respective calibration files wereused to calculate the treatment times for a certain dose. This was mentioned to the company
engineer who had installed the system. The engineer was of the view that the local physicists
did not measure the absorbed dose correctly for verification of the calibration files using a
locally created phantom.
(99)

Some time later, in connection with a meeting on other technical issues, the matter

was brought up again with the company. The company realized that they had provided anincorrect calibration file for one of the 4 cm applicators, causing the dose to differ by +20%
from the intended dose. Due to the low energy of radiation (50 kV), only 1.5 to 2 mm extra
tissue got irradiated with an excessive dose.
Discussion and lessons:(100)

The most important point from this event is that, when discrepancies in dose

measurements are found, it is the ultimate responsibility of the hospital to thoroughly
investigate why these deviations are occurring, before applying the beam to real patient
treatments. However, it is also the responsibility of manufacturers, suppliers and installers todeliver the correct equipment with the right calibration files and accompanying documents,
for which an effective internal quality control is needed to discover errors before theMINIMIZING THE RISK OF UNINTENDED EXPOSURE FROM NEWER RADIATION

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THERAPY TECHNOLOGIE

equipment is handed over, and to provide correct information and advice, upon questions
formulated by the hospital staff.
(101)

Furthermore, the supplier bears responsibility for delivering equipment-specificinformation for its safe use (including measurement geometry used when creating calibration
files). A lesson that suppliers could learn from this event is the need to ensure training fortheir engineers on the tests to be performed and documented, before and during acceptance,
including the issue of calibration files and the advice given to the hospital staff.
(102)

The reliance of the physicist on an informal opinion of the engineer suggests

additional lessons for the hospital, i.e., the need for including in the purchase contract, the listof acceptance tests to be performed, and a procedure to follow up on discrepancies during
acceptance, as well as during commissioning and afterwards.
4.1.3.

Beam output drift in tomotherapy

Case 3: Incorrect tolerance for the interlocks of a tomotherapy machine (SLH 2008)(103) On a tomotherapy machine, on which daily morning checks were systematically
performed to assess the beam output stability, a sudden drift was observed one morning, withan underdosage larger than 10%. The internal safety interlocks of the machine were not« seeing » this drift. Further treatments of patients were cancelled, although there was a doubt
on the validity of the response of the local dosimeters used for these measurements that
seemed to differ from the response of a dosimeter supplied by the manufacturer.
(104)

After further investigation, it appeared that the deviation between the local and the

manufacturer’s dosimeter was only 1.3% but that the safety threshold used for the interlock of
the output of the tomotherapy machine had been set previously with a tolerance larger than
±10%. The examination of the log-book of treated patients revealed that three patients treatedin the afternoon of the day preceding the faulty morning check were underdosed by 12 %. It
was later discovered that the incorrect setting of the interlock was there from the original
installation of the machine. Apparently, the installation technician enlarged the tolerance tofacilitate the adjustment of the beam output and forgot to set it back to the correct values. It is
still unclear why the interlock was set with such a wide tolerance and why there had been asudden drift of the machine output (but the magnetron and the target were replaced after the
problem was discovered, before the machine was restarted for new treatments).

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Discussion and lessons(105)

The output verification of a radiation therapy machine is performed according to apredefined schedule (typically daily). The dose delivery for all patients treated between two
consecutive checks relies on the stability of the machine and its dose monitoring system. In
principle, internal safety interlocks should prevent incorrect output but they are set according
to a certain tolerance which is not usually accessible to the user or checked as part of the local
internal QA procedures. If the tolerance is too large and if a problem occurs with the machine
output between two consecutive checks, it could remain undetected until the next check. For atomotherapy machine the dose rate is critical because it is used in combination with the tabletranslation to control the delivered dose. However, similar problems can be found on aconventional linear accelerator e.g. with respect to the beam uniformity or monitor responsecontrol.
(106)

The lesson of this incident is that the users need to understand how the beam ismonitored and which interlocks are provided by the manufacturer. This need may have to belinked to the acceptance tests. With the assistance of the manufacturers, the users should
include in their internal QA procedure a method to check that the safety interlocks are set
properly, especially after maintenance or repair. In addition, manufacturers should develop
more advanced solutions, preferably automated, to avoid machine parameters being set
outside of the allowed range.
4.2. Issues related to treatment preparation
4.2.1.

Problems with dynamic wedges

Case 4: Monitor unit calculation for the wrong type of wedge (ASN, 2007a, SFPM, 2006, Ash,2007, Derreumaux, 2008)(107)

A new treatment technique was being introduced in 2004 at a hospital, when it

was decided to change from static mechanical (hard) wedges to dynamic (soft) wedges for thetreatment of prostate cancer patients. When treating with open fields or using hard wedges inthis center, the standard practice was to independently verify monitor units throughcalculation checks, as well as using diodes for an independent check of the dose delivered.
The physicist involved in the change of technique at the time was the only physicist working
at the facility, and was also on-call in another facility.

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MINIMIZING THE RISK OF UNINTENDED EXPOSURE FROM NEWER RADIATIONTHERAPY TECHNOLOGIE(108)

As part of the introduction of the new treatment technique, two of the dosimetrists(TPS operators) were given two brief demonstrations on how to use the software. Whenchanging treatment technique, the previous safety barriers of independent calculation ofmonitor units and verification with diodes were removed. The reasons for this were that theindependent calculation software was not adapted to dynamic wedges and the interpretation ofdiodes results would have been much more difficult when using soft wedges compared towhen using hard wedges.
(109)

The terminology of the treatment planning software, including displays, was inEnglish, as were the operator manuals. Not all the French operators correctly understood theabbreviated English display on the interface to the treatment planning system and they
mistakenly selected hard wedges identified with their angle when intending to select the planwith soft wedges. The correct box to tick in the treatment planning system software for soft
wedges was indicated “EW” (enhanced dynamic wedge) without any angle indication, which
was not easily understandable and did not correspond to the terminology in French. When thetreatment planning of a patient had been finalized, and the dose distribution had been
optimized with hard wedges, the parameters, including the monitor units (MU) were manuallytransferred to the treatment accelerator where the dynamic wedge option was manually
selected. The calculated number of monitor units for hard wedges that were used was muchgreater than the number of monitor units that were needed to deliver the same absorbed dosewith soft wedges. Consequently, the patients affected by this error received a higher absorbed
dose than intended. The reason for the higher number of MU is illustrated in Figure 4.1 andTable 4.1 below.

MINIMIZING THE RISK OF UNINTENDED EXPOSURE FROM NEWER RADIATIONTHERAPY TECHNOLOGIESFig 4.1 In this particular accelerator, when the dynamic wedge function is used, at the beginning of theirradiation the collimator is almost closed and the jaws are aligned on the field edge where the higher
dose is to be delivered (right side of the figure).. As the irradiation begins, the moving jaw starts moving
towards the opposed edge of the field (left on the figure). This is analogue to a hard wedge with its thinedge aligned to the right field edge. A physical wedge in a symmetric field, however, is centred with thebeam (see lower part of the figure) and is designed to cover the largest field size. Thus, the thin edge ofthe physical wedge does not coincide with the field edge, except when the largest field size is selected.
Therefore, for this particular accelerator design, it is only for the largest field size that the attenuation of
the physical wedge would be equivalent to the dynamic wedge and the ratio of MU between hard andsoft wedge would be close to unity. For all smaller field sizes the attenuation of the phyisical wedge ishigher than the dynamic wedge and the ratio of MU is > 1. The smaller the field size the higher the MU
ratio, as shown in table 4.1 below.

Enhanced dynamic (soft) wedge in this particular accelerator isequivalent to a shifted physical wedge (variable attenuation at beamaxis depending on the field size)

Physical wedge centred with field. The attenuation
at beam axis is always the same, and the value isdictated by the maximum design field size

53

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MINIMIZING THE RISK OF UNINTENDED EXPOSURE FROM NEWER RADIATIONTHERAPY TECHNOLOGIE

Table 4.1 Ratio of the numbers of monitor units (MU) for physical / dynamic wedge, for thesame dose delivered at 15 cm depth, isocentric technique, physical wedge 45º

6 MV

20 MV

Ratio of MU forphysical / dynamic
wedge

Ratio of MU forphysical / dynamic
wedge

5 cm x 5 cm

1.83

1.83

10 cm x 10 cm

1.55

1.61

15 cm x 15 cm

1.32

1.43

20 cm x 20 cm

1.12

1.27

Field size

Adapted from Rosenwald, Institute Curie, France(110)

Between May 2004 and August 2005, at least 23 patients received an overdose of

20-35% more than the intended dose. Between September 2005 and September 2006, four
patients died as a result of this accident. At least ten patients showed severe radiation
complications, with symptoms such as intense pain, discharges and fistulas. Regional
authorities were informed the month following the accident, but national authorities only
received information a full year after the accident had taken place.
Discussion and lessons(111)

Two “brief demonstrations” to two dosimetrists are not sufficient to move fromstatic wedges to the newer technology of dynamic wedges, an important increase incomplexity, and a critical safety issue. Rather, more thorough and effective training, isrequired. The difference in the number of MU between static and dynamic wedges, in thisparticular accelerator design, was not fully appreciated. This insufficient understanding was
aggravated by the removal of some check provisions such as independent calculations ofmonitor units and dose checks with diodes. In summary, insufficient understanding of a new,more complex technique, inadequate user interface, instructions and displays in a language
not understood by operators, and removal of checks made the accident more likely.
(112)

One important lesson is that there may be a temptation to remove checks, when

they can not be applied to a new technology in a straightforward manner. This is a challenge

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THERAPY TECHNOLOGIES

inherent in more complex technology. The safety philosophy is to increase supervision at theimplementation of a new technique and to maintain the required level of safety, even if it
entails the design of appropriate verifications or adapting the old ones, but the decision should
never be to compromise safety.
4.2.2.

Computer problems with intensity modulated radiation therapy

Case 5: Computer crash and loss of data in IMRT planning (CP, 2005, NYC-DHMH, 2005)(113)

A patient with head and neck cancer (oropharynx) was going to be treated withintensity modulation radiation therapy (IMRT) at a radiation therapy facility in March 2005.
An IMRT plan was prepared, as per standard protocol. A verification plan was also preparedand tested prior to treatment (as required in by the QA programme of the hospital) in order toverify that the calculated dose distribution would be achieved at irradiation. This verification
plan confirmed the dosimetric correctness of the plan through use of portal dosimetry.
Subsequently the treatment of the patient was delivered correctly for the first four fractions.
(114)

After these fractions, a physician reviewed the case and came to the conclusionthat the dose distribution needed modification in order to reduce the dose to specific organs at
risk. This task was given to a dosimetrist who started by copying the treatment plan in order
to work with modifications on the copy. Re-optimization started, where existing fluenceswere deleted and new fluences were optimized to follow the new instructions for an optimaldose distribution. When completed, these new fluences were saved to the database. As thenext step in the generation of the new plan, final calculations were started. In this step themultileaf collimator (MLC) motion control points were generated in order to guide the MLC
motion for achieving the desired dose distribution through the IMRT treatment. This was
performed correctly, and a new digitally reconstructed radiograph (DRR) was also obtained.
(115)

The new treatment plan was now complete, and the final step was to save the plan

to the database. A “Save all” process started. The items to be saved were (1) the newly
generated actual photon fluence data; (2) the new DRR; and (3) the new MLC control points.
When saving the items to the database, the data is first sent sequentially to a holding area on
the server. It is not until all items have been received at this holding area that they will besaved permanently in the database.
(116)

The actual fluence data was saved to the holding area, but when the DRR was

being saved a problem occurred. An error message appeared on the treatment planningsystem, indicating that the data could not be saved. This “transaction error message” read:

56

MINIMIZING THE RISK OF UNINTENDED EXPOSURE FROM NEWER RADIATIONTHERAPY TECHNOLOGIE“Please note the following messages and inform your Systems Administrator: Failed to accessvolume cache file <C:\Program Files\...\504MImageDRR>. Possible reasons are ... disk full.
Do you want to save your changes before application aborts? Yes. No”. The operator pressed“Yes”, which began a second, separate, save transaction. The DRR was, however, still locked
in to the faulty first save transaction, causing the second save transaction to be unable tocomplete the process whereby the software appeared to be frozen.
(117)

In reconstruction of the event, it appears that the operator then tried to terminate

the software application manually, either by pressing ctrl-alt-del or through the Windows task
manager. This manual termination would have caused the database to perform a roll-back to
the last valid state, which in this case contained the newly created actual fluence data, which
had been saved in the first save transaction, and an incomplete part of the newly created DRR
from the same save transaction. However, since the saving of items is sequential, there was no
file containing the MLC control points which should have been saved after the DRR.
(118)

A few seconds later, the operator calls up the patient’s treatment plan on another

TPS workstation. Since the new fluence had been saved, the operator was able to calculate thenew dose distribution and save this. This could be done regardless of the fact that there was
no MLC control point data saved.
(119)

According to the QA procedure in the clinic, the next step should have been to

produce a new “verification plan” (sometimes called “hybrid plan”) and perform in-phantom
measurements to verify the consistency of the dose distribution achieved at irradiation. Also
according to the QA procedure of the facility, a physicist should have reviewed the newtreatment plan prior to irradiation of the patient. The “verification plan” was not calculated at
this time, and it is unclear if a physicist reviewed the plan independently. Had these steps
been taken it would have been noticeable that the irradiated area outline lacked an MLC
shape, both in the TPS and at the treatment console, thus using an open field.
(120)

The patient was treated with the incorrect plan, i.e., an open field, for threefractions. Due to the higher number of monitor units that the MLC collimator shaped fieldwould have required, the overdose to the patient treated with an open field became
substantial. The patient received 39 Gy in 3 fractions to the head and neck area.
(121)

Only after the third fraction, was a “verification plan” created. When this plan wastested on the treatment unit, the absence of the MLC became apparent and the accident was
revealed.

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Discussion and lessons(122)

There are four levels at which this type of problem can be detected: 1) whenplanning the new treatment configuration there is an opportunity to detect that the dynamic
MLC option was not set as intended; 2) an independent review of the plan by a second
dosimetrist or a physicist could also detect this type of error; 3) when doing the treatment
preparation for the initial set up it could be seen on the treatment console that the unit was notdoing what was expected (i.e. no MLC-leaf movement for an IMRT-patient) and 4) a“verification plan”, as prescribed by the quality control programme of the hospital would
definitely discover the wrong dose and dose distribution. A computer “crash” is not an
uncommon event, but in radiation therapy treatment planning and delivery it can be very
dangerous. Analysis of the potential effect of a computer crash needs to be integrated in thesafety assessment and a procedure needs to be in place for the radiation therapy staff tosystematically verify data integrity after a computer “crash”.
(123)

In summary, even when a QA programme exists, it can be rendered inefficient if

work is performed somewhat “mechanically”, and QA procedures are ignored in somesituations. It is also possible that the staff applies the procedures correctly for new treatment
plans, but fails to do so for a change of the treatment plan. To reduce this type of problem,
heads of radiation therapy departments and hospital administrators need to provide continuous
encouragement to “work with awareness” and supervise compliance with QA procedures, notonly for the initial treatment plan but also for treatment modifications.

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4.2.3.

Errors in imaging for radiation therapy treatment planning

Case 6: Reversal of MRI images (NRC, 2007)(124)

When preparing the treatment of a patient’s brain tumour at a clinic, an MRI studyof the brain was undertaken. Standard practice was to position and scan the patient “headfirst”, i.e. entering the scanner with the head first, and then import the scans into theGammaknife treatment planning system for optimization of absorbed dose, dose distribution
and treatment geometry. However, for this patient, due to the “feet first” scan technique being
selected in the software of the imaging unit, the right and left sides of the brain weretransposed in the MRI images. When importing the images into the TPS, this was not noticed.
The treatment planning that was subsequently performed thus targeted an incorrect location in
the brain. As a result, the patient received a high radiation dose to the wrong side of the brain.
Discussion and lessons(125)

It seems that the imaging staff performing the MRI scan was not aware of theneed for an accurate scanning and recording protocol, including image orientation when
imaging for the radiation therapy department, in this case, “scanned head first”. There are two
measures to avoid this type of error: 1) to have clear instructions visibly posted with written
protocols known to and followed by the imaging staff, when imaging for radiation therapy
treatment planning and 2) to include in the quality assurance program, procedures forverifying ‘left from right’ in safety critical images, e.g. by using fiducial markers, whereappropriate.
4.2.4.

Treatment set up errors from virtual simulation markers

Case 7: Confusing set-up markers and tattoos when introducing virtual simulation [ROSIS,
2008a](126)

During the early stages of introducing virtual simulation into a clinic, a breast

cancer patient was undergoing this new simulation technique. The intention was to simulate astandard two-field tangential isocentric treatment set-up. The personnel were used toconventional simulation, where this isocentre is determined at the time of simulation. In
virtual simulation, the treatment isocentre location is not known at the time of scanning.
(127)

At the time of the virtual simulation scanning of this patient, tattoos for lining up

the patient (set-up tattoos) were marked on the patient’s skin. The intention was to use thesetattoos to indicate the origin of the CT coordinate system (reference point). In the subsequentMINIMIZING THE RISK OF UNINTENDED EXPOSURE FROM NEWER RADIATION

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treatment planning, the offset from the CT origin reference point to the isocentre of thetreatment was determined and noted in the patient’s treatment chart.
(128)

When the patient came for the first treatment, the staff at the treatment unit

misunderstood what the tattoos indicated, and thought that they were indicating the locationof the treatment isocentre. As a result, the patient was treated in a couch position 3 cm below
the intended couch position.
The treatment procedure indicated that a check of the source-to-skin distance should be
performed. The different distance to the breast with the patient shifted 3 cm in the axial
direction should have been noticed, but this verification was not done. Electronic portal
images of the field placements were taken at this first fraction. These were compared withdigitally reconstructed radiographs (DRR) and seen and approved by the physician, who was
not used to seeing digitally reconstructed radiographs as reference images. The treatment
procedure also indicated that the physicist should have compared the couch axial position
from the treatment plan with the actual couch position in the treatment room at the patient setup, but this was overseen.
Discussion and lessons:(129)

The radiation therapy technologists did not seem familiar with the different

meaning of the tattoos used for virtual simulation and misunderstood them. A lesson from thisevent is that new procedures for using virtual simulation should be introduced only with
sufficient training, including exercises for all relevant staff groups, until it can be ensured thatthey understand them and are aware of critical aspects. As with any new technique, whenintroducing virtual simulation, it is important to follow the QA programme, in particularquality control procedures. In this case, skipping the check of SSD at treatment and theverification of axial couch position made the accidental exposure more likely.
4.2.5.

Digitally reconstructed radiograph errors

Case 8: Geometrical distortion of digitally reconstructed radiographs (CIB, 2007)(130)

Digitally reconstructed radiographs (DRR) are often created in the treatment

planning process to be used as reference images for the intended set-up of the patient inrelation to the treatment unit isocentre and beam placement. The geometric integrity ofreference images is of great importance. At the same time, the underlying algorithms for DRRMINIMIZING THE RISK OF UNINTENDED EXPOSURE FROM NEWER RADIATION

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reconstructions in a treatment planning system are difficult to verify for the clinical end-userof the system.
(131)

A specific treatment planning system used several methods in parallel for thecreation of DRR. For one of these methods, which was introduced with an updated version of
the treatment planning software, an error was detected which resulted in incorrect formationand display of the DRR when certain conditions were fulfilled. The problem originated in anerror in how the information from the CT slices was loaded into the graphics memory of the
TPS computer, causing the volume to be stretched out in the z-axis in comparison with thescale of the actual CT series. As a result, there could be a positioning error equal to theminimum distance between slices in the CT series, or in some cases up to twice this distance.
(132)

Since the DRR exported by the system could have the same problem if createdusing the faulty method, the error would then propagate to the incorrect geometric set-up of
the patient if this incorrect DRR was used as reference image.
Discussion and lessons(133)

A software update (in this case, involving DRR images) is as important as new

software or new equipment and should be tested in the factory and also properly
commissioned at the hospital. The manufacturer can lower the probability of delivering faulty
software by performing stringent software tests in the factory, challenging the system in asystematic way in these tests. The hospital needs to select, plan and carry out a subset of therelevant commissioning tests on the TPS and data transfer. However, a problem that appears
only occasionally, when certain conditions are fulfilled, tends to escape tests and verifications.
This type of problem is suitable for sharing among users and manufacturers and lessons andcommissioning approaches should be disseminated in a timely manner.
4.3. Events related to patient data management
4.3.1.

Errors when using “record and verify” systems

Case 9: Incorrect manual transfer of treatment parameters (SMIR, 2006, Mayles, 2007,
Williams, 2007)(134)

In May 2005, the record and verify system at a hospital was upgraded to a morecomprehensive electronic patient data management system. Previously, the transfer oftreatment parameters had been performed manually, while after the upgrade the system could

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perform these transfers electronically. This was implemented for most, but not all, treatment
procedures in the clinic.
(135)

Towards the end of 2005, a young patient came to the hospital with a relatively

rare brain tumour (pineoblastoma) and it was decided to give this patient a radiation treatment
to the whole central nervous system. The absorbed dose prescribed was 35 Gy in 20 fractions
to the whole CNS, followed by 19.8 Gy in 11 fractions to the site of the tumour. The
craniospinal radiation therapy was prescribed to be performed using two lateral fields
covering the brain, matched with an upper and a lower spine field. This type of treatment wasconsidered to be of a complex type, and it was only performed about six times per year in theclinic.
(136)

As part of the quality assurance procedures in the clinic, dosimetrists 4 werecategorized as belonging to one of five categories ranging from the most junior category to
the most senior. At the same time, treatment plans were categorized as belonging to one offive categories ranging from the simplest to the most advanced plans. Contrary to approved
procedures, a junior dosimetrist was given the task of developing this advanced treatment
plan, with the opportunity to be supervised by a more senior dosimetrist when creating theplan. There are indications that this supervision was not an active supervision, but had a morereactive nature to queries from the person being supervised. The junior dosimetrist did nothave queries to the supervisor and seems to have been unaware of some of the complexities in
the plan.
(137)

With the new procedure for automatic electronic transfer, monitor units to givethe prescribed dose to the dose prescription point are calculated directly in the treatment
planning system. With the old procedure, monitor unit calculations were done by the planningsystem, firstly by giving the absorbed dose 1 Gy to the normalization point, and subsequently
scaling it up to the prescribed dose by manual multiplication with the dose per fraction. The
craniospinal treatment technique was one of the few techniques that had not yet been includedin the new procedure of automatic electronic transfer. Instead, the treatment planning was
performed according to the old procedure.

4

The meaning of the word “dosimetrists” is not uniform throughout the world; it is used in this report to mean
the person who performs the treatment planning and clinical dosimetry.

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MINIMIZING THE RISK OF UNINTENDED EXPOSURE FROM NEWER RADIATIONTHERAPY TECHNOLOGIE(138)

When planning the left and right lateral fields covering the brain in the treatmentof this particular patient, the junior dosimetrist let the TPS calculate the MUs for theprescribed dose, as in the new procedure, instead of calculating the MUs to deliver 1 Gy as in
the intended old procedure. The junior dosimetrist then transferred this MU setting to themanual planning form which was passed to a radiation therapy technologist for manual
calculations of the MU. This manual planning form contained the MUs corresponding to theprescribed dose instead of 1Gy. The technologist performed the subsequent up scaling of themonitor units according to the old procedure, which meant that the number of MU was now
67% too high for each of the lateral head fields. It should also be noted that the dosimetrist
had made a second error when filling out the MU settings in the manual planning form, in thatthe figure for the total number of fractions had been wrong in the calculations, leading to the
67% overdose, instead of 75%, according to the ratio of 1.75 Gy to 1 Gy. This error was notfound by the more senior planner who checked the calculations.
(139)

In the resulting treatment, the patient received 2.92 Gy per fraction to the headinstead of the intended 1.75 Gy. This went on for 19 fractions, when the same juniordosimetrist committed the same error with another plan. This was spotted by the checker ofthis new treatment plan and the original error was found. The patient died nine months after
the accident.
(140)

It should be noted that radiation therapy physics staffing levels in Scotland wereless than 60% of the recommended levels at the time of the accident, and that staffing levels
in treatment planning in the clinic also were well below the level recommended by theprofessional body in that country.
Discussion and lessons(141)

The initial comment to make is that using two different methods to transfer data,with the associated opportunity for errors, should be avoided as much as possible. If there arestrong reasons to keep the manual transfer of treatment parameters for some treatment
procedures, and if a type of treatment is used only a few times a year, it may be sensible toalways assign the treatment to the same person (or two persons). The next reflection points toa real challenge for hospital administrators: the challenge of ensuring a working environmentthat facilitates alertness, due thought and compliance with procedures. Relaxation seems tohave occurred at two different levels at this hospital; 1) assignment of the advanced task to ajunior planner, contrary to hospital procedures, and 2) failure of a calculation check by the

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senior dosimetrist. An important lesson is that this occurred in spite of the quality assurance
programme in place, with a sophisticated staff structure, grouped in five different categoriesof treatment planners.
4.4. Events related to treatment delivery and treatment verification
4.4.1.

Significant radiation exposure from electronic portal imaging

Case 10: Excessive exposure by the daily use of electronic portal imaging (Derreumaux,
2008)
(142)

Since the introduction of electronic portal imaging devices (EPID), clinics havethe opportunity to monitor the set-up of individual patients more readily than with portal
films. While a modern EPID can display a portal image with a short and, therefore, low
exposure of the patient, typically a few monitor units, some of the earlier models of EPID
required a much longer exposure to form an image.
(143)

A clinic, the same one as in Case 4 above, had installed the type of EPID that was

based on a matrix of liquid ionization chambers and had decided that the set-up position ofpatients treated for prostate cancer should be verified daily. In order to do this, they took two
daily single exposure images for each patient (i.e. one anterio-posterior and one lateralimage). In addition to these daily positional images, once a week portal images were taken foreach patient and for all fields to confirm the correct placement of the irradiation fields. The
weekly portal images were performed using a double-exposure technique where the sameanatomical parts were exposed twice.
(144)

As part of the clinic’s practice, when performing the weekly double exposure

verification of the treatment beams, the MUs used for the irradiation field were deducted from
the MUs to be used for the treatment in order to give the intended absorbed dose. However,
the increased exposure arising from the daily positional images was not considered in therecording of the dose given to the patient, nor the part of the weekly double exposure of thepatient outside the irradiation fields.
(145)

The EPID in the clinic required a relatively high exposure for an electronic portal

image. As a result, it has been estimated that each patient received a daily absorbed dose of
between 0.15 and 0.20 Gy in excess to the prescribed dose due to the electronic portal
imaging routines. In total, 397 patients were affected between 2001 and 2006 in that they

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received an absorbed dose between 8 and 10% higher than intended. All patients affected by
the overdose in Case 4 were also affected by this case, thus adding to the already substantial
overdose of those patients.
Discussion and lessons(146)

There seems to have been unawareness of the magnitude of the absorbed dose

resulting from frequent use of the imaging system used for setup, and that this dose could besignificant with respect to the total treatment dose. This unawareness seems to be the reason
for not taking the radiation dose from daily imaging into account. Before introducing newimaging technologies and verification procedures into clinical practice, procedures forassessing the exposure from imaging should be required.
4.4.2.

Errors with stereotactic radiosurgery field size

Case 11: Incorrect field size used for stereotactic treatment (Derreumaux, 2008)(147)

A clinic was using a linear accelerator for stereotactic treatment of intracranial

targets using a set of additional cylindrical collimators with opening diameters ranging from10 to 30 mm mounted on an opaque supporting tray to be attached to the accelerator’saccessory holder. For the correct use of these cylindrical collimators, it was necessary to set
the collimator aperture to a size of 4 cm x 4 cm.
(148)

When treating a patient with arterio-venous malformation (AVM) with a single

fraction, the additional cylindrical collimator for stereotactic treatment was attached to thelinear accelerator. The operator was verbally instructed by the physicist to narrow thecollimator aperture to “40 40”, but instead of using the field size 40 mm by 40 mm, theoperator used the field size 40 cm by 40 cm.
(149)

As a consequence of this, the fully opened field was applied to the patient throughthe brass tray supporting the additional cylindrical collimator, which would only cause a very
limited attenuation of the beam, thereby giving nearly a full absorbed dose to large areas
outside the target volume.
(150)

When evaluating the magnitude of overexposure locally, the impact wasunderestimated, leading to an incorrect assessment of the severity of the clinicalconsequences, which were not fully appreciated or provided for.
(151)

The clinical consequences attributed to this were fibrosis and osteo-tracheal (???

oesophago-tracheal) fistula which led to a surgical intervention and subsequently death of the

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patient from hemorrhage. The very severe clinical consequences do not seem to match thedose calculated for this volume suggesting the potential presence of an additional problem.
Discussion and lessons(152)

The following reflection can be made from this event: when the operator heard“40 40”, he/she associated it to a conventional radiation therapy field of 40 cm x 40 cm,
instead of thinking of radiosurgery. Furthermore, a cylindrical collimator of a few mm
diameter, inside the “40 cm x 40 cm beam” did not appear strange to him/her or it did not
trigger a question. Thus the operator seems to have been performing a new technique stereotactic radiation therapy, without full awareness of this technique. The next question iswhether there was a well documented prescription and treatment procedure, given the fact thatthe instruction on the field size was verbal, and its misunderstanding caused the accidental
exposure. In summary, there seems to be a combination of insufficient training in the newtechnique, and work based on a verbal instruction instead of a written prescription ortreatment plan. Furthermore, the local evaluation of the impact of the accident seems to havebeen inadequate, and point to the usefulness of calling for external expert guidance when
unusual irradiation conditions in an accidental exposure are being evaluated.
4.5. References
Ash, D., (2007) Lessons from Epinal. Clin Oncol 19:614-615
ASN (2007) Report concerning the radiation therapy incident at the university hospital centre(CHU) in Toulouse – Rangueil Hospital. ASN – Autorité de Sûreté Nucléaire
ASN (2007a) Summary of ASN report n° 2006 ENSTR 019 - IGAS n° RM 2007-015P on the
Epinal radiation therapy accident. G. Wack, F. Lalande, M.D. Seligman
Customer Infromation Bulletin (Nucletron) (2007): Incorrect creation of DRR using GPUbased methods. CIB-OTP 192.047-00.
CP (2005) [Treatment Facility] Incident Evaluation Summary, CP-2005-049 VMS. 1-12.
Derreumaux, S., Etard, C., Huet, C., Trompier, F, Clairand, I. Bottollier-Depois, J.-F., Aubert,
B., Gourmelon, P., (2008) Lessons from recent accidents in radiation therapy in France.
Rad Prot Dosim xx

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THERAPY TECHNOLOGIE

Mayles, W.P.M. (2007) The Glasgow incident – a physicist’s reflections. W.P.M. Mayles.
Clin Oncol 19:4-7.
NRC (2007) Gamma knife treatment to wrong side of brain. Event Notification Report 43746.
United States Nuclear Regulatory Commission
NYC-DHMH (2005) ORH Information Notice 2005-01. Office of Radiological Health, NYC
Department of Health and Mental Hygiene.
ROSIS(2008),

Radiation

Oncology

SafetyInformation

System(ROSIS)

http://www.rosis.info
ROSIS (2008) Radiation Oncology Safety Information System (ROSIS) http://www.rosis.info
SLH (2008) Saint-Luc Hospital, Belgium, internal report.
SFPM (2006) Accident de radiothérapie à Épinal. P.J. Compte. Société Française de Physique
Médicale
SMIR (2006) Accidental overexposure of patient Lisa Norris during radiation therapy
treatment at the Beatson Oncology Centre, Glasgow in January 2006. Report of aninvestigation by the Inspector appointed by the Scottish Ministers for The Ionising
Radiation (Medical Exposures) Regulations 2000.
Williams, M. V. (2007) Radiation therapy near misses, incidents and errors: radiation therapy
incident in Glasgow. M.V. Williams. Clin Oncol 19:1-3.

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5. PROSPECTIVE APPROACHES TO AVOIDING ACCIDENTAL EXPOSURES(153)

While valuable lessons can be learnt from the detailed analysis of incidents andaccidental exposures which have occurred, these lessons are limited to reported experience.
There may be non-reported incidents or potential incidents which have not yet happened but
are possible. Such accidental exposures can only be avoided if they are anticipated. In
addition, increased complexity places new demands on quality assurance programmes, andthe approach of following all-inclusive test lists and all possible controls may become
impractical within the context of limited resources. Therefore, with technological and processchanges, retrospective approaches are not sufficient and all-inclusive quality control checks
may not be feasible. There is, thus, a need for proactive, structured, and systematic
approaches to the identification of system weaknesses and the anticipation of failure modes,
evaluating and comparing potential risks from each identified failure mode. Such approaches
should allow a rational selection of the checks to be performed and facilitate the distributionof resources in a manner, likely to be most beneficial to the patient.
5.1. Treatment process flow diagrams.
(154)

The identification of weaknesses in the system requires an understanding of thesystem itself. A helpful approach to the understanding of the system is through visualization
by means of a process flow diagram. The generic process through which a patient passes inany encounter with a health care system includes the following five steps: assessment,prescription, preparation for treatment, treatment delivery and follow up (Ekaette, 2006).
Patient data flows from one step to the next with return loops as necessary. A feature whichcharacterizes modern radiation therapy is an electronic patient data management system(PDMS) which may form part of the electronic medical record and links together
electronically all or most of the processes involved in the five steps above. It is the availabilityof such systems capable of transferring large amounts of data that has enabled new treatment
strategies such as IMRT and IGRT to be introduced into the clinic.
(155)

The intermediate three steps of prescription, preparation and delivery are those of

most interest in this document. Each of the major steps can be divided into substeps. For
example, preparation for treatment includes patient immobilization, image segmentation andstructure delineation, calculation of three dimensional dose distributions and machine settings

68

MINIMIZING THE RISK OF UNINTENDED EXPOSURE FROM NEWER RADIATIONTHERAPY TECHNOLOGIE(such as monitor units) and the transfer of data to the patient data management system. Should
it be useful for the analysis, these substeps may be further subdivided. For example, the dose
calculation substep includes the specification of objectives and constraints used in the
optimization of the plan. An example of a process flow chart for the three intermediate steps
in radiation therapy has been presented by Rath (2008).
(156)

An alternative representation of the activities taking place prior to and during acourse of radiation therapy is known as a process tree. The trunk of the tree conducts thepatient from entry into the system to successful completion of the treatment. The boughs
connected to the trunk represent the tasks, such as immobilization, image segmentation andstructure delineation and treatment planning, which are necessary for completion of thetreatment. Along each bough the substeps, such as choice of fusion and segmentation
algorithms and margin selection, are identified. An example of a process tree for HDR
brachytherapy has been presented by Thomadsen (2003).
The clinical processes, visualized by means of the clinical process flow diagram or clinicalprocess tree, are carried out using the clinical infrastructure of the institution (hardware,
software, documentation, etc.). Establishing and maintaining the clinical infrastructure of an
institution also involves processes which are principally acceptance testing, calibration,commissioning and ongoing quality control. Process flow diagrams describing activities
related to the maintenance of the clinical infrastructure are also useful in the identification of
failure modes. Past experience indicates that it is infrastructure failures that generally have themost significant consequences as their effects are systematic and they can affect large
numbers of patients.
(157)

The importance of acknowledging the possibility of failures of both infrastructure

and processes can be seen from the examples in Chapter 4. For example, although each step inthe patient process may be carried out correctly it is possible that the infrastructure is faulty,
due for example to an error in calibration (Case 1 in Chapter 4). Failures of processes
involved in the commissioning of clinical infrastructure can lead to very high severity
incidents in which many patients will be potentially affected. Similarly, it is possible that
although each relevant component of the clinical infrastructure is performing as expected, theclinical process is flawed with severe consequences for a large number of patients (Case 4).
(158)

Clinical process flow diagrams and trees are graphical representations, broadly inchronological order, of the activities required to be performed for the successful completionMINIMIZING THE RISK OF UNINTENDED EXPOSURE FROM NEWER RADIATION

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THERAPY TECHNOLOGIESof treatment of a patient. They may also be inferred from the several classification schemes
used for clinical incident reporting and analysis (for example: “Towards Safer Radiation
therapy”, https://www.rcr.ac.uk/membersarea/shop/layout6.asp?PID=261&Child=Child). As
mentioned above it may be helpful to separately construct an infrastructure process tree which
describes the calibration, commissioning and on-going quality control of equipment,
maintenance and release of clinical data and procedures together with any other components
required for the treatment of all or a cohort of patients.
(159)

The process flow diagram or tree developed for use in an individual clinicalprogram should reflect the structure of that program and the sequence of activities taking
place in a manner that is logical and clear to the multidisciplinary team responsible for thecare of the patient. A balance needs to be struck between simplicity, to maintaincomprehensibility, and complexity, to capture all possible failures of the system. An approachto the validation of local process diagrams and trees is the assessment of their ability tocapture all the historical incidents described in this and other relevant documents as far as
they are applicable to local circumstances. This is a necessary but not sufficient condition for
the adequacy of the representation. In the case of new technologies and treatment strategies,
additional processes over and above those identified from historical events will need to beincorporated.
5.2. Process flow diagram and the design of Quality Assurance programmes(160)

Process flow illustrations can facilitate the design of quality assurance

programmes. Each process or group of processes should be recognized within the quality
assurance programme. Key components of quality assurance in radiation therapy are thecommissioning and re-commissioning of infrastructure and clinical processes and regularquality control. The quality control program needs to encompass infrastructure (for example
monthly checks of a linear accelerator), clinical processes (for example, checks of calculatedmonitor units for individual treatments) and, increasingly, patient specific activities (forexample, experimental verification of the fluence distribution for IMRT beams). An agreed
upon process flow illustration is helpful in the design of a comprehensive, effective andefficient quality assurance programme.

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5.3. Failure modes(161)

Process flow diagrams and trees facilitate the next step in a prospective approach

to risk management which is the identification of possible failure modes at each step in theprocess. As this is a prospective approach, prior statistically valid experience is likely to belimited. The approach normally adopted for the identification of failure modes is to convene
an expert panel which reviews the process flow illustrations and, within a structured context,
uses their judgment to prepare a list of possible failure modes. There are two major challenges
in completing this step. The first is to be confident that all possible significant failure modes
have been identified. In meeting this challenge the range of experience of the expert panel
will be important. As a minimum, possible failure modes should include those reported in theliterature and on public databases. The second challenge is describing the failure modes
unambiguously. Not only must the description of the failure mode be completely clear but thesame failure originating from different sources need to be differentiated. A failure in somestep that led to a particular clinical consequence that could arise from equipment malfunction
or human error needs to be identified as two failure modes. The clinical outcome might be thesame but the likelihood of occurrence and, importantly, remedial measures could be quite
different.
(162)

The amount of detail and the degree of specificity in the description of a failuremode is the analyst’s decision. However, to be useful the description should be sufficient toguide any process or quality assurance changes that result from the analysis. Examples offailure modes attributed to the historical accidental exposures described in Chapter 4 are givenin the Appendix.
(163)

Having identified, on the basis of global experience and the opinion of local

experts, ideally with input from equipment manufacturers, potential failure modes, the next
step is to assess the risk associated with each failure mode.
5.4. Risk(164)

For the purposes of this document, risk can be regarded as some function of theprobability of an event’s occurring and the severity of the consequences for the patient should
the event occur. Severity indices may also reflect the numbers of patients involved.

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The diagram below illustrates how risk might be assessed. The graph represents

an idealized frequency distribution of delivered doses, when there are no systematic errors
present, with the central peak indicating the radiation oncologist’s prescribed dose. Somevariability in the relationship between the prescribed and delivered doses is inevitable due touncertainties in calibration of the treatment machine, algorithm limitations in the treatment
planning system and patient positioning reproducibility over a course of treatment requiring
30 or more fractions. It is possible using clinical data and observation to determine a range ofacceptability for the delivered dose (Mijnheer et al. 1987). At some deviation from theprescription, the treatment becomes unacceptable in terms of the negative consequences forthe patient. Although overdosing often attracts more attention, inadvertent underdosing canclearly also have major consequences for the patient. The thresholds at which catastrophic
clinical events occur will depend on the clinical situation. However, in order to track
significant accidental exposures some regulators have defined the threshold to be at a specificvalue over a course of treatment (for example, the Nuclear Regulatory Commission in the
United States (USNRC), http://edocket.access.gpo.gov/cfr_2003/10cfr35.3045.htm). The
region between acceptable and unacceptable treatments could be described as sub-optimal.
There may not be clear clinical evidence that treatments falling in that region result indeleterious effects on the patient. However, significant departures from (evidence based)prescription are clearly not desirable.
(166)

It is important to note that the diagram describes the situation in which there areno systematic errors causing dose deviations from prescription, such as an error in either thecalibration (Case 1) or use of the technology (Case 4). When systematic deviations are present
the curve will no longer be centered on the oncologist’s prescription and hence the frequencyof either under or overdosing will be increased. . If there is a systematic error affecting all
patients, the whole curve would shift to the right (overdose) or left (underdose), by theamount of the deviation (see Figure 3). There may also be systematic deviations affecting
only certain types of treatments, and therefore affecting only part of the patients, which is also
represented in fig xx. The end result will be that systematic deviations will result in larger
numbers of patients exceeding the threshold of acceptability and by a larger amount thus
enhancing the severity associated with systematic errors (ICRP 2000).
(167)

In the diagram, the abscissa could be four dimensional dose, i.e. including

fractionation in time. A distribution could also be constructed to reflect inadequacy infollowing the oncologist’s prescription for the volumes to be irradiated and, equally

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MINIMIZING THE RISK OF UNINTENDED EXPOSURE FROM NEWER RADIATIONTHERAPY TECHNOLOGIE

important, the volumes not to be irradiated. It is noted that volume in this context is also four
dimensional as its location within the patient may change with time due to, for example,
respiration. One of the newer technologies under active investigation is gated therapy for lungcancer. Loss of synchrony between the target motion and time of irradiation can have severe
deleterious effects. Enhancing safety is interpreted as reducing the mass density of the
distribution in the regions of unacceptability. If the distribution is not centered on the
oncologist’s prescription, systematic effects are present and must be rectified. If the
distribution is Gaussian or close to it, shrinking the tails will also narrow the peak,corresponding to reduced variability and hence an improvement in quality for a centered
distribution.
5.5. Three prospective approaches(168)

Once the failure modes have been identified, the task becomes that of assessing

the probability of an unacceptable event’s occurring (the first contribution to the ordinate in
the diagram), assessing the severity or consequence of the event should it occur (the abscissa),
and in some of the approaches, assessing the likelihood that the event, should it occur, will
not be caught during quality control procedures (the second contribution to the ordinate) andhence have a negative impact on the patient’s treatment. The prospective approaches of risk
assessment described below employ methods of analyzing the problem in this way.
(169)

The three prospective approaches which are most commonly used are failuremodes and effects analysis (FMEA) (Stamatis, 1995), probabilistic safety assessment (IAEA
2006) and risk matrix (RM) (Ortiz et al., 2008). They are not totally independent, as FMEA is
often used as the first step to probabilistic safety assessments, as described later in thisdocument.
5.5.1. Failure mode and effect analysis(170)

An example of the application of a Failure Modes and Effects Analysis toradiation therapy is that performed by Task Group 100 of the American Association of
Physicists in Medicine (Huq, 2008). Three numerical values were used to describe eachfailure mode. O (for occurrence) describes to the probability that a particular untoward event
occurs. S (for severity) is a measure of the severity of the consequences resulting from thefailure mode if it is not detected and corrected. D (for detectability) describes the probability
that the failure will be detected before the treatment commences or the failure is effective. In

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the TG100 implementation, O ranges from 1 (failure unlikely, < 1 in 104) to 10 (highly likely,
more than 5% of the time). S ranges from 1 (no danger, minimal disturbance of clinicalroutine) to 10 (catastrophic if persists through treatment). D ranges from 1 (very detectable 0.01% or less of the events go undetected throughout treatment) to 10 (very hard to detect,>20% of the failures persist through the treatment course). An important point to note in theevaluation of detectability, D, is that the failure mode is assumed not to have been detected
through routine quality control within the sub-process of occurrence. Thus, in TG 100’s
implementation of FMEA, the likelihood of (lack of) detection at any point further
downstream from the sub-process in which the failure occurred is estimated.
(171)

Multiplying these three numbers together yields a Risk Probability Number (RPN)

which can be used for prioritizing quality control tests and activities.
(172)

The presentation of a complex and comprehensive FMEA is challenging. An

instructive approach is to incorporate the findings of the FMEA within the process flow
diagram or process tree which was developed at the initiation of the analysis. For example,
failure modes with risk probability numbers (RPN) greater than a certain threshold might behighlighted in a process tree. This diagram then contains the essential elements of a fault tree.
Fault trees can be constructed using either or both retrospective and prospective analyses andthus are a flexible and useful tool in safety analysis.
(173)

The calculation of risk probability numbers implies knowledge of the functional

relationship between probabilities of occurrence and detectability and the severity of theconsequences in quantifying risk. Institutional judgment may dictate that higher severity
events, even with a relatively low probability of occurrence, deserve increased attention. Sucha policy can also be incorporated into the fault tree by displaying failure modes the severity ofwhich exceeds some threshold irrespective of the probability of occurrence/detectability.
5.5.2. Probabilistic safety assessment(174)

Probabilistic Safety Assessment (PSA) is a prospective tool that has beensuccessfully used in the aeronautics, nuclear and petrochemical industries, but it has also been
proposed for use with radiation sources in industry and in medicine (ICRP, 1997). PSA
provides safety assessments in an exhaustive and structured way by combining the effects ofequipment faults, procedural and human errors and hence provides insights into the strengths
and vulnerabilities of the process being studied, the dominant contributors to the overall risk
and options to reduce it.

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5.5.2.1.

Example of an application of PSA to radiation therapy treatment with a linear

accelerator.
(175)

An example of the application of a PSA to treatments with linear accelerators(ESTRO Congress 2008, IRPA 12, 2008) is that performed by a task group of the Ibero
American FORO of Nuclear and Radiation Safety Regulatory Agencies. The task group was
multidisciplinary (radiation oncologists, medical physicists, technologists, regulators,
maintenance engineers and specialists in probabilistic safety assessments). The study was
devoted to conventional treatments with accelerators, as a first step to progression to newer
technologies.
(176)

In this study, equipment failure modes and human error analysis was used toobtain an exhaustive list of deviations with a reasonable probability of occurrence and whichmight produce significant adverse outcomes. Accident sequences were graphically modeledby means of fault trees and event trees. After combining the appropriate models, a Boolean
reduction was conducted by computer software to obtain sequence “cut sets”, i.e. theminimum combination of equipment faults and human errors which produce a given
accidental sequence. This step was very important in finding common-cause event sequences.
(177)

Given the scarcity of statistical data on reliability of equipment and human errors

available from radiation therapy, generic data bases from several sources (AAPM 1993, NRC
1995 a and b) were used to estimate the reliability of equipment as it is typically
recommended for topical PSAs that are applied for the first time. For human error
probabilities, screening values were used, i.e. conservative values which allow filtering themost important human actions and focusing efforts on them in further detailed analysis. Thisapproach allows for relative analysis from the absolute results obtained, since the wholequantification was done using the same type of self-consistent data.
5.5.2.2.
(178)

Summary of results
As many as 443 failure modes, which could potentially cause the postulated

undesired events, were analyzed by the failure modes and effects analysis (FMEA A summaryof these failure modes, and their relative impact is given in the following paragraphs.
5.5.2.3.
(179)

Accidental exposures involving single and multiple patients
Common-cause analysis of the probabilistic safety assessment has shown that as

few as eight different event sequences are responsible for 90% of the potentially catastrophic

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accidental exposures involving multiple patients. These events are summarized in thefollowing table
Table 5.1 . Individual and cumulated contribution of failure modes andhuman errors to the total frequency of accidental exposures involving
multiple patients
Individual

Cumulative

contribution

contribution

45.5%

45.5%

32%

77,5%

3.9%

81,4%

2.7%

84%

2.4%

86.5%

1.9%

88.4%

Missing out in vivo dosimetry 5, where applicable

1.5%

89.9%

Skipping portal imaging at the initial treatment session or

1,4%

91,3%

Failure mode or human error

Using a TPS in a different manner from the instructions
for use, modification of its procedures, without validating
the modified procedure
Omitting external beam revalidation in the monthly
quality control of the TPS or doing it incorrectly.
Neglecting medical control of patients during the courseof treatments or overlooking abnormal signs
Missing out independent checks of the dosimetric plan or
doing it inefficiently, i.e., overlooking errors.
Missing out patient observation by the radiotherapy
technologists during daily treatment or overlooking
abnormal signs.
Missing out portal imaging or obtaining a poor image, notsuitable for the purpose.

performing it incorrectly.

(180)

The most vulnerable step of the treatment process is treatment planning, with a

93% contribution to the total probability of accidental exposures involving multiple patients.
The high contribution stems from potential human error in the use of treatment planning5

For more complex technologies, such as IMRT or tomotherapy in vivo dosimetry is not used, but dosimetricchecks by phantoms irradiation are done instead.

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systems (TPS). If treatment planning systems are used in a different way from what is written
in the instructions, it is possible to introduce errors in the calculation of doses and dose
distributions, which may escape software interlocks and system warnings. If the error is maderepeatedly, i.e., making a change in the instruction, in written or non written form, and themodified procedure is not validated it may cause an accidental exposure of multiple patientssimilar to that which occurred in Panamá, in 2000-2001 (IAEA, 2001).
(181)

Single patient accidental exposures are much more likely than multiple patient

accidental exposures. Single patient accidental exposures amount to 98 % of all patient
accidental exposures. This quantification is a significant contribution from the probabilistic
safety assessment. Although much more attention has been devoted so far to reports ofcatastrophic-type, low-probability accidental exposures involving multiple patients, otherevents, with less catastrophic, but still significant consequences, are much more likely tooccur. They may be underreported but also deserve attention.
(182)

The contribution of each step in the treatment process to the total risk of single

patient accidental exposure shows that the most vulnerable steps in the treatment are
misunderstanding the delineated treatment volumes (59%), followed by initial treatment
session (21%), patient positioning for daily treatment (15%), daily treatment delivery (9%),
and treatment planning (8%).
(183)

The most important events involving single patient accidental exposures are

human errors at the volume delineation step, which may consist of mistaking as gross tumor
volume what is meant to be the clinical target volume or vice versa on the TPS, and omitting
one or several secondary target volumes or one or several organs at risk. The higher risk
associated with these actions is the fact that, for this particular type of errors by the radiation
oncologist when delineating treatment volumes, there are hardly any redundant or
independent safety measures after the error. It can perhaps be detected by the discussion
between radiation oncologist and the medical physicists at the treatment planning phase and at
the approval of the treatment plan. The efficiency of such opportunities is limited because it ismainly the same person who made the mistake who can detect it.
Analysis of importance: risk increase and risk reduction factors(184)

Probabilistic safety assessment includes tools to perform sensitivity analyses, inorder to obtain knowledge on how important a given element is, i.e. how much would the riskincrease if the element were to fail or not be present, and how much would be the risk

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reduction if the element were never to fail. This information is crucial for risk-informed
decision making and rational allocation of resources. In this short summary, only a few
examples are given
1. Software is responsible for controlling multiple functions and is assumed to bereliable. Changing the assumption to postulate a software fault would mean anincrease of the risk of multiple patient accidental exposures, by a factor of 30 to300, depending on the system and the type of fault. For this reason, development ofsoftware safety standards and evidence of compliance with these standards are
essential. Regulation and further standardization of software development andtesting are needed.
2. The ‘record and verify’ system of medical accelerators drastically reduces the riskof nine initiating events related to daily treatment delivery. Absence of this systemincreases the risk by a factor of 75, according to the computations of probabilities
made in this research. New equipment should, therefore, include ‘record and verify’systems.
3. Mistakes related to irradiating ‘the wrong patient’ can be avoided by bar codes orby assigning a space for a photograph of the patient on treatment cards, onelectronic treatment sheets and on CT forms. Absence of this provision can
increase the risk of single patient accidental exposures by up to a factor of 18.
Patient positioning mistakes can be reduced by assigning a specific place in thetreatment sheet for a picture of the initial patient setup.
4. The presence of two technologists during treatment preparation and delivery
permits one of them to be dedicated to double check patient preparation andpositioning. Absence of this measure increases the risk of accidental exposure by afactor of 10. At least one of the two technologists should be the same during thewhole course of treatment, from the initial setup until the end of the treatment.
5. . Independent review of the TPS calculation substantially reduces the risk of single
patient accidental exposure. Absence of this safety measure increases the risk by afactor of 10.

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5.5.3. Risk matrix(185)

The risk matrix approach is kept much simpler, as compared with probabilistic

safety assessments, in order to be usable in each individual radiation therapy department. The
quantitative assessment of probabilities is replaced by a simpler, semi quantitative, four-level
scale (for example, very low, low, high and very high) and the complex algebraic analysis of
the event sequence done in probabilistic safety assessment is replaced by a logicalcombination of the four levels of the frequency of the initiating event, the likelihood of failureof the safety provisions and the severity of the consequences. This combination results in a
global risk for each event. The result in term of risk is also four-level scaled. This part is used
as a screening process to filter out negligible-risk events and focus on the higher risks.
(186)

The next step is a deeper analysis of the event sequences, that have been “labeled”

by the risk matrix as having unacceptable risk (high or very high). The deeper analysis
addresses the effectiveness of the existing safety provisions and the need for additional
measures. For example, in the screening process the likelihood of failure of the set of safetymeasures is estimated from the number of measures only, while in the deeper analysis, theirstrength is checked by asking, for each event sequence with unacceptable risk, the following
questions: “can the safety provisions be considered robust enough for their estimatedprobability of failure to be reclassified as “low”?”; “can the frequency of occurrence of theevent sequence or its consequences be reduced?”; “is there a need to add one or more safety
provision to reduce the risk to an acceptable level (low or very low)?”. The answers to thesequestions provide corrections to the risk matrix and constitute the conclusions andrecommendations from the study.
5.6. Closing the loop and applying prospective methods.
(187)

The objective of the application of any of these prospective analyses is theminimization of the risk of accidental exposure to patients. The methods of prospective
analysis discussed above, anchored as they are in process flow diagrams and process trees,suggest how improvements in safety might be implemented. A given failure mode will result
in consequences of a given severity. Estimates of severities of failure modes provide an
opportunity for prioritising safety improvement initiatives. The analysis also provide
estimates of the likelihood of a failure occurring and the likelihood of its being detected. Overboth of these probabilities the institution does have some control. If a prospective analysis has

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identified a failure mode with a high probability of occurrence this suggests that the activities
involved warrant re-examination. Linking the analysis to the clinical or infrastructure process
flow illustrations identifies which process or processes are involved. Where a weakness is
identified, either prospectively or retrospectively, that process requires revision with theintention of minimizing the probability of occurrence. Similarly, failure modes associatedwith low detectability suggest that quality control procedures need to be re-examined. With aquality assurance program built on the foundation of a process flow illustration, the activities
which may require closer scrutiny will also be more readily identified.
(188)

Whether a quantitative (FMEA and probabilistic safety assessment) or a morequalitative (RM) prospective analysis is undertaken the results will facilitate prioritisation ofremedial measures based on overall estimated risk to the patient or severity or some othercriteria selected by the institution.
(189)

It appears from the foregoing that, full prospective analyses are complex and time

consuming. National and international organizations and professional bodies can assist
individual clinics in recommending easily interpretable scales for quantifying occurrence,
severity and detectability and by illustrating the use of prospective techniques applied togeneric process trees (Huq 2008). Developers of patient safety databases, while primarily
facilitating retrospective analyses, can, through the thoughtful design of their databases,
facilitate prospective analyses as historical information is useful for validating, in part,process illustrations.
(190)

To summarize, prospective analyses are an essential component of a safetyassessment, particularly for technological and process changes and are, in addition, a usefulapproach to managing risks with existing equipment and current work practices. A properly
conducted prospective analysis helps identify potential failure modes and the severity of theensuing clinical consequences. Where a particular failure mode is associated with a high riskof occurrence then the infrastructure component or clinical process needs to be revised, redesigned and conducted accordingly. When the prospective analysis indicates that a particular
failure mode is unlikely to be detected then the quality control programme and safetymeasures require strengthening.

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Frequency

(191)

Dose
Figure 3a. Frequency distribution of the ratio of delivered to prescribed dose in the absence of systematic

relative number of patients

effects.

1

2

d e live re d d o se , n o rm a lize d to p re scrib e d va lu e

Figure 3b.. Frequency distribution of the ratio of delivered to prescribed dose, in presence of a systematicdeviation affecting approximately one third of the patients. This case is similar to the event occurredbecause of a modified use of the TPS in which only some patients treated in the abdominal area wereaffected (reference Panama)MINIMIZING THE RISK OF UNINTENDED EXPOSURE FROM NEWER RADIATION

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5.7. References

AAPM, 1993. American Association of Physicists in Medicine, Medical Accelerator
SafetyConsiderations. Report No. 56, Medical Physics, Vol 20, Issue 4..
Derreumaux, S., Etard, C., Huet, C., Trompier, F, Clairand, I. Bottollier-Depois, J.-F., Aubert,
B., Gourmelon, P., (2008) Lessons from recent accidents in radiation therapy in France.
Rad Prot Dosim xx
Duménigo, C, Ramírez, M.L.,

Ortiz López, P., McDonnell, J.D., Vilaragut, J.J.,

Papadopulos, S., Pereira, P., Gonçalves, M., Morales, J., Ferro, R., López Morones, R.,
Sanchez, R. , Delgado, J.M., Sanchez, C., Larrinaga, E., Somoano, F., Guillén, A.,
Rodríguez, M. (2008). risk analysis methods: Their importance for safety assessment ofpractices using radiation. IRPA 12.
Ekaette, E. U., Lee, R.C., Cooke, D.L., Kelly, K.L., Dunscombe, P. (2006) Risk analysis inradiation treatment: application of a new taxonomic structure. Radioth. Oncol. 74, 282287
ESTRO (2008) European Society for Therapeutic Radiology and Oncology. Annual
Congress.
Huq, M.S., Fraass, B.A., Dunscombe, P.B., et al. (2008) A method for evaluating quality
assurance needs in radiation therapy. Int. J. Radiat. Oncol. Biol. Phys. 70, S170-S173
IAEA (2001). Investigation of an Accidental Exposure of Radiotherapy Patients in Panamá.
International Atomic Energy Agency. IAEA, Vienna.
IAEA (2006) Case studies in the application of probabilistic safety assessment techniques toradiation sources. International Atomic Energy Agency. IAEA Tec-Doc 1494 IAEA,
Vienna, Austria.
ICRP (1997). International Commission on Radiological Protection.. Protection from
Potential Exposure: Application to Selected Radiation Sources, ICRP Publication 76.
Annals of the ICRP 27 (2), (Pergamon Press, Oxford).
ICRP (2000) International Commission on Radiological Protection.. Prevention of accidental
exposure of patients undergoing radiation therapy, ICRP Publication 86. Annals of the
ICRP 30(3), (Pergamon Press, Oxford).

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MINIMIZING THE RISK OF UNINTENDED EXPOSURE FROM NEWER RADIATIONTHERAPY TECHNOLOGIE

Mijnheer, B.J., Battermann, J.J., Wambersie, A. (1987) What degree of accuracy is requiredand can be achieved in photon and neutron therapy? Radioth. Oncol. 8, 237-252
NRC, 1995 a. United States Nuclear Regulatory Commision NUREG/CR –6323 UCRL-ID12005. Relative Risk Analysis in Regulating the Use of Radiation – Emitting Medical
Devices. A preliminary Application.
NRC, 1995 b. United States Nuclear Regulatory Commision NUREG/CP –0144 INEL94/0111 A Workshop on Developing Risk Assessment Methods for Medical Use ofRadioactive Material, USA, 1995
NRC (1988 and 1996). Nuclear Regulatory Commission. Report to Congress on Abnormal
Occurrences. AS-88-8. Multiple Medical Teletherapy Misadministration at Sacred Heart
Hospital in Cumberland, Maryland. NUREG-0090, Volume 11, No. 4 and 19.
Washington DC,.
NRC (2007) Gamma knife treatment to wrong side of brain. Event Notification Report 43746.
United States Nuclear Regulatory Commission
Nucletron (2007) Incorrect creation of DRR using GPU-based methods. Nucletron Customer
Information Bulletin CIB-OTP 192.047-00.
Ortiz López, P., Duménigo, C, Ramírez, M.L.,

McDonnell, J.D., Vilaragut, J.J.,

Papadopulos, S., Pereira, P., Gonçalves, M., Morales, J., Ferro, R., López Morones, R.,
Sanchez, R. , Delgado, J.M., Sanchez, C., Larrinaga, E., Somoano, F., Guillén, A.,
Rodríguez, M. (2008). Radiation safety assessment of cobalt 60 external beam radiation
therapy using the risk-matrix method. IRPA 12, 2008
Patton, G. A.; Gaffney, D. K. & Moeller, J. H. (2003), 'Facilitation of radiotherapeutic error
by computerized record and verify systems.', Int J Radiat Oncol Biol Phys 56(1), 50--57.
Rath, F. (2008) Tools for developing a quality management program: proactive tools (process
mapping, value stream mapping, fault tree analysis and failure mode and effects analysis).
Int. J. Radiat. Oncol. Biol. Phys. 70, S187-S190
ROSIS (2007), Report no. xx. Radiation Oncology Safety Information System (ROSIS)
http://www.rosis.info
ROSIS (2007a) Report no. xx. Radiation Oncology Safety Information System (ROSIS)
http://www.rosis.info

MINIMIZING THE RISK OF UNINTENDED EXPOSURE FROM NEWER RADIATION

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THERAPY TECHNOLOGIESStamatis, D.H. “Failure modes and effects analysis” 1995, American Society for Quality
Control, Milwaukee, WI.
Vilaragut Llanes, J.J., Ferro Fernández, R., Rodríguez Martí, M., Ortiz López, P., Ramírez,M.L.,

Pérez Mulas, A., Barrientos, M., Somoano, F., Delgado Rodríguez, J.M.

Papadopulos, S., Pereira, P., López Morones, R., Larrinaga Cortina, Rivero Oliva, J.J.,
Alemañy, J., (2008), Probabilistic Safety Assessment of radiation therapy treatment
process with an electron Linear accelerator for medical uses. IRPA.

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6. CONCLUSIONS AND RECOMMENDATIONS(192)

This chapter is a summarized recapitulation of the main points made in Chapters 2

and 4 on lessons from accidental exposure and near misses, from the safety implications of
new technologies as anticipated in Chapter 3 and from prospective approaches to safetyassessments treated in Chapter 5.
6.1. Complexity(193)

Substantial improvement in radiation therapy has been achieved in association

with increasing complexity and the omnipresence of computers. As a result of complexity,“human common sense” may no longer be as effective a mechanism to perceive “whensomething may be wrong” as it is with conventional radiation therapy. The number ofpossible quality control tests increases to the extent that it may become unaffordable.
Increased complexity requires a different strategy to deal with risks.
(194)

The primary responsibility remains with the user for the safe application of new

and existing technologies and treatment strategies. However, other parties having subsidiary
responsibility can, and should, assist the user in meeting this responsibility. The manufacturer
should, through pre-release testing, identify both limitations in performance and pathways
which may lead to the misuse of their equipment. Updated information, as experience isgained, needs to be rapidly disseminated to users.
•

Increased complexity requires a strategy of combining•

Manufacturer design of safety interlocks, alerts and warnings, self-test
capabilities for equipment and easy-to-understand user interfaces in alanguage understandable to the user and adherence to international
standards to ensure compatibility between equipment from different
manufacturers.

All these safety measures are equally important forsoftware.
•Education and equipment-specific training for new technologies, withformal involvement of manufacturers. Such training should address theunderstanding of warnings and interlocks provided by manufacturers, inparticular those related to beam monitoring. Such warnings and interlocks
should be considered in developing acceptance tests and quality control

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procedures.
•

Risk-informed and cost-effective approaches for prioritizing tests andchecks by means of prospective methods of risk assessment, to be performedin cooperation with manufacturers.
6.2. Dose escalation(195)

Tumour dose escalation requires a reduction of geometrical margins, in order toavoid an increase of normal tissue complication probability. This is only feasible with an
improvement in the dose conformality, accurate and precise patient positioning andimmobilization and by image guided irradiation. It also requires a clear understandingof the overall accuracy achieved for patients in clinical practice in order to safely reduce
margins. Without these features, tumour dose escalation could lead to severe patientcomplications.
6.3. Increased role of imaging(196)

Radiation exposure from imaging which, until recently was considered negligible

as compared to the therapeutic radiation exposure, is becoming significant due to its increased
use. In addition, the dose distribution from imaging may be different from that of thetreatment, as a result of the different beam energies and field sizes used for imaging andtreatment. The fact that different beam characteristics are used during treatment preparationand delivery involves a risk of confusing them and giving the wrong dose and dose
distribution if the wrong beam characteristics are used in the calculation (Case 10, Chapter 4,
and Derreumaux, 2008)(197)

The increasing number of different types of imaging modalities requires

identification of patient orientation when passing through the gantry of the imaging
equipment (CT, MRI, virtual simulation); “head first” or “feet first” stays consistent throughall steps, i.e., from image acquisition, treatment planning to treatment delivery, as thisbecomes a critical issue (see Chapter 4, Case 6 and NRC, 2007). In addition, a risk fromgeometric distortion and wrong tissue density arises from the direct use of CT numbers for
subsequent dose calculation, especially because of artifacts and the presence of contrast media(Chapter 4, Case 8 and CIB, 2007).

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MINIMIZING THE RISK OF UNINTENDED EXPOSURE FROM NEWER RADIATIONTHERAPY TECHNOLOGIE•

When introducing new imaging technologies into radiation therapy practice,assessment of imaging doses becomes necessary and they should be properly
integrated and combined in treatment planning and delivery. Procedures are needed
for recording image orientation with respect to the patient ensuring consistency
through the whole process from prescription to delivery, and for choosing thecorrect images and CT numbers for tissue density estimation, giving specificattention to possible image artifacts and possible geometric distortion.
6.4. Increased demands on purchasing, acceptance and commissioning(198)

New technologies are not different from conventional radiation therapy

approaches with respect to the propagation of errors in commissioning to all patients, but theincrease in complexity makes it more challenging to design comprehensive commissioning
programmes that would catch all potential pitfalls.
•

A programme for purchasing, acceptance testing and commissioning should not
only address the treatment machine but also increasingly complex treatment
planning systems, “record and verify” systems, imaging equipment used forradiation therapy, software, procedures and entire processes.
(199)

Case 2 in Chapter 4 (ROSIS, 2008) shows that reliance on an informal opinion by

the installation engineers has lead to an accidental exposure, related to the delivery of a wrong
calibration file. When dose deviations are found, the hospital or clinic is responsible forinvestigating and understanding the cause of deviation, before applying the beam to clinicalpatient treatments.
•

Hospital staff should stay aware of the fact that the overall responsibility for thecorrect absorbed dose determination and the correct treatment of the patientsremains with the user in hospital. This includes investigating discrepancies in dose
measurements, before applying the beam to patient treatments.
(200)

On the other hand, subsidiary responsibility lies with manufacturers and suppliers.

They should also have an internal quality assurance programme encompassing the training ofservice engineers and the tests to be performed and documented, including the issue ofcalibration files.
•

Manufacturers and suppliers, however, have subsidiary responsibility for delivering
the correct equipment with the correct calibration files and accompanying

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documents, and for providing correct information and advice, upon request from thehospital staff. They should, therefore, have a policy and procedures to provide
correct advice, upon questions formulated by relevant hospital staff, especially thosequestions related to deviations and discrepancies.
•

Independent checks of measurements and calculations continue to be a vital safety
layer.

6.4.1. Re-commissioning of modifications and updates(201)

A software update related to DRR images led to an incident (Case 8 in Chapter 4

and CIB, 2007). Modifications or software updates are as important as new software or new
equipment and they need not only tests in the factory but also proper commissioning at thehospital.
•

There is a need for re-commissioning of the relevant devices after equipment
modifications and software updates and also to monitor impact on related processes.
(202)

A problem that occurs only rarely, when certain conditions happen to coincide,tends to escape tests and verifications (Case 8 in Chapter 4 and CIB, 2007).
•

Timely and effective sharing of operational experience is critical in the phase of
introducing new techniques and technologies and especially necessary for problems
that appear only seldom, when certain conditions happen to coincide, because thistype of problems tends to escape ordinary tests.

6.4.2. Specific safety issues related to treatment planning systems(203)

Complex modern TPSs offer so many functionalities with so many possible

pathways that it becomes extremely important to follow formal acceptance testing procedures(IAEA, 2004, 2007) and to check system functions, document different capabilities and verify
the accuracy of dose calculation algorithms. Commissioning of a TPS is also very critical.
The wrong input of basic parameters may lead to severe systematic errors involving many
patients. A misunderstanding related to the use of a TPS can lead to severe consequences asshown by reported accidental exposures. Many failures would result in a system crash with noother consequences but in some peculiar circumstances some failures lead to a compromised
outcome (Case 5 in Chapter 4, and CP, 2005, NYC-DHMH, 2005).
•

It is advisable to use proactive safety assessment approaches, as described in thisdocument, to identify circumstances potentially leading to accidental exposure and to88

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choose safety measures in a selective manner. Although users are responsible forcommissioning such systems, proactive safety assessment should be preferably donewith the help of experienced colleagues and/or in the framework of structured
networks or user groups, and manufacturers should alert them about thecharacteristics of the measurements involved.
6.5. Omnipresence of computers(204)

One of the most important features of “new technologies” is the omnipresence ofcomputer based solutions. Computers are used at each stage of the process, from prescription
to completion of the treatment. Simple traditional displays on the console are now replaced bydialogue with the machine, which may be in a language not fully understandable to theoperators. This becomes more important if there are abbreviations in another language. An
example of this was the abbreviation EW, for enhanced dynamic wedges (Case 4 in Chapter
4, ASN, 2007a, SFPM, 2006, Ash, 2007, Derreumaux, 2008). In addition, computer crashes
are not uncommon, but in radiation therapy they can be extremely dangerous if not managed
properly (Chapter 4, Case 5, NYC-DHMH, 2005).
•

Equipment

instructions

andhuman-machinecommunication

shouldbeunderstandable to the users. Procedures should be in place to deal with situations
created by computer crashes, which may cause loss of data integrity. These
procedures should include a systematic verification of data integrity after acomputer crash, occurring during data processing or data transfer.
(205)

The use of computers has an impact on the “state of mind” of the staff. As an

example, technologists who know that there is an R&V verify system working in the
background, inevitably tend to relax attention in comparison with a manual system which
entirely depends on their actions. There are possibilities of misregistration in electronic
patient charts, that should be carefully investigated, but it is difficult to review the numerous
possible pitfalls related to their introduction.
•

It is necessary to develop procedures and to plan a commissioning and a “probing”
period when introducing an electronic patient chart, until it is confirmed that such asystem can be used safely.

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6.6. Education and training(206)

Errors in calibrating small radiation beams from micro collimators and the error ofmisunderstanding of field size in radiosurgery have shown that new technologies require
deeper knowledge and understanding of the science and techniques involved (Case 1 in
Chapter 4, ASN, 2007, Derreumaux, 2008). When a new technology or technique is being
introduced, education and training needs revisiting at three levels: 1) education on the proper
understanding of the physics involved in the process, 2) “hands-on” training to obtain thenecessary expertise before being allowed to use the new techniques “alone”, and 3) specifictraining in the equipment and techniques to be used.
There is a danger of underestimating the required training associated with an increase incomplexity, and there may be a temptation to oversimplify the issue by replacing proper
training with a short briefing or demonstration, from which important safety implications of
new techniques cannot be fully appreciated (Case 4 in Chapter 4 and ASN, 2007a, SFPM,
2006, Ash, 2007, Derreumaux, 2008).
•

The following conclusion for conventional radiation therapy from ICRP 86 (ICRP
2000) is equally applicable, and even more relevant and important, for new
technologies: “purchasing new equipment without a concomitant effort on educationand training and on a programme of quality assurance is dangerous”. The lessons
from reported accidental exposure have confirmed that embarking on newtechnologies makes revisiting staff qualifications necessary. In particular, a solid
understanding is required, for example, the physical effects of a partial irradiation ofa detector if it were larger than the beam cross-section (micromultileaf collimators);
dynamic wedges and their impact on the delivered dose for different accelerator
designs; the labeling, recording and interpretation of imaging parameters; thecomplex functionalities of treatment planning systems; the particular requirementsof stereotactic radiosurgery, such as the field sizes and collimator settings involved.
6.7. Use of proactive safety assessment in designing a quality assurance programme(207)

Due to increasing complexity, quality control as implemented with conventional

technologies may become impractical because of the extensive checklists aimed at measuring
everything possible. Quality assurance programmes, including lessons learned from reported

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accidental exposures and near misses, are very important but are not sufficient to identify
other possible accidental exposures, which occurred but were never reported or latent risks
which never manifested themselves and hence remain unknown. This is especially true for
new technologies, for which the time is too short to have enough operational experience in theform of reports. These problems can be overcome with a combination of the following tools:•

The programme of checks should be rationalized and simplified with the help of
manufacturers, by designing proper alerts and warnings, self-test routines, especiallyrelated to software, easy-to-understand user interfaces, internal safety interlocks aswell as training in the proper and cautious use of the equipment.

•

Increased complexity requires a strategy to choose control checks, based on selective,risk-informed, approaches to identify and prioritize tests. In cooperation with
manufacturers, mechanisms should be found to perform proactive safetyassessments when a new product, technology, or technique is being introduced.

•

Timely and effective sharing of operational experience is crucial in the phase of
introducing new techniques and technologies. This could be achieved by organized
and structured sharing mechanisms, for example, creating moderated electronic
FORUMs and by early establishment of panels of experts.
(208)

With these recommendations in mind, there are a number of more specific issues

which are summarized below.
6.7.1. Change in processes and staff responsibilities(209)

As shown in Chapter 3, new technologies lead to the changing of processes,

procedures and also the shifting of staff resources. An example is that of patient positioning
and selection of parameters which may be simplified by a modern record and verify systemand data transfer, but treatment planning becomes more sophisticated. The introduction of
new technologies and techniques might also affect surrounding processes not directly part of
the new technology.
•

Changes in processes, procedures and task allocation also need to be commissioned
and regularly quality controlled. The full potential impact of such changes should beassessed.

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6.7.2. The problem of tests that are no longer effective(210)

Conventional checks may not be easily applicable to complex treatments such as

IMRT, for example, the manual check of the number of monitor units. An important lesson
from one of the accidental exposures described in Case 4, Chapter 5, is that there is a
temptation to remove safety checks, which were carried out for conventional techniques,because they can not be easily applied to new technologies.
•

When conventional tests and checks are not applicable or not effective for new
technologies, the safety philosophy is to find measures to maintain the required levelof safety. This may need the design of new tests or the modification and validation of
the old ones. A conscious decision is required to avoid compromising safety.

6.7.3. Consistency in prescription(211)

Intensity modulated radiation therapy (IMRT) and inverse planning haveintroduced significant changes in the approach to prescribing radiation therapy. Withoutestablishing protocols, lack of consistency between treatments may result in substantial
deviations, when the same intended prescription is expressed in different forms (Chapter 3,
Das, 2008).
•

Protocols for prescription, reporting and recording, such as ICRU reports, should be
kept updated to reflect and accommodate new technologies and adopted at a national
level. Professional bodies can be instrumental in achieving this need.

6.7.4. Coordinates, reference marks and tattoos(212)

It is very important to maintain consistency of co-ordinate systems following

through all the steps of the treatment chain. In case 7 the radiation therapy technologist did
not seem familiar with the different meaning of the markers and tattoos used for virtual
simulation and misunderstood them (Case 7 in Chapter 4, and ROSIS 2008a).
•

Procedures for virtual simulation, and their implications in the whole treatment
chain, should be introduced with sufficient training to ensure confidence that thestaff is familiar with them and aware of all the critical aspects. A consistentcoordinates system is required for the whole process from virtual simulation throughtreatment planning to delivery.

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6.7.5. Handling of images(213)

Images play an important role for the accuracy of treatment delivery and safety

and errors could lead to significant consequences for the treatment outcome, since the error
may affect the whole course of treatment, from the beginning to the end. An example is theattachment of the reference image to an incorrect beam or vice versa. Another type of error islaterality in which the labeling of images, such as left-right sides, is incorrect (Chapter 4, Case
6 and NRC, 2007).
•

Written instructions should be visibly posted and followed by the imaging staff,
which performs the imaging for radiation therapy treatment planning and delivery.
These instructions should include procedures for verifying left and right in criticalimages, e.g. by using fiducial markers.

6.7.6. Uniformity and clarity in data transfer approaches(214)

Keeping more than one method in the department for planning patient treatment

and transferring data (e.g. automatic and manual) to the accelerator is often inevitable, but it isa potential source of error with the risk of applying a procedure to a case for which it is notintended (Case 9 and SMIR, 2006, Mayles, 2007, Williams, 2007).
•

When several methods and different protocols for data transfer are used for treating
patients in a given department, the patient categories to which the protocols areapplicable should be clearly defined and communicated, including details about
which planning system and which data transfer method is applicable.

6.7.7. Safe communication among interdisciplinary professionals(215)

Loose communication, combined with other human errors, triggered theaccidental exposure described under Case 11, which ended with the death of the patient. The
field size of “40 x 40” was taken to be in “cm” instead of “mm”. It was probably taken for
granted that the technologist was familiar with typical field sizes used in stereotactic
neurosurgery and assumed that he/she would correctly interpret the meaning. Although there
was apparently an added problem of the technologist not being trained and familiar withradiosurgery, the role of correct communication should never be underestimated. Loose
communication can also lead to ignoring, or to insufficient investigation of, unexpectedradiation effects on patients, as has occurred in some accidental exposures in conventional

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radiotherapy, thus minimizing the possibility of mitigating the consequences of accidental
exposure.
•

Communication should follow a given pattern regarding content and format, as well
as formal recording of safety critical issues. Unambiguous communication isessential, especially considering the complexity and the multidisciplinary nature ofradiotherapy.

6.7.8. Maintenance, repairs and notification of the physicist(216)

A severe accidental exposure with conventional techniques showed it is necessarythat a report produced by a maintenance engineer after a repair is seen by the physicist, who
can judge whether or not the work done may affect the beam characteristics or the beam
output and can then perform the required checks (IAEA, 2000, CRP, 2000).
•

Procedures to notify a physicist of maintenance or repair activities were identified as
crucial in conventional technology, but are even more necessary in new complex
technologies, in which modifications, software updates, adjustments and calibration
files can be introduced in computer dialogue among various devices and might goundetected, unless formally notified.
6.8. Safety culture(217)

Lack of awareness of the differences implied by new techniques, and thinking interms of conventional techniques, combined with an ambiguous verbal communication led toa fatal misunderstanding (IAEA, 1998, 2000, ICRP, 2000). Case 9 in Chapter 4 (SMIR, 2006,
Mayles, 2007, Williams, 2007) shows that a programme of quality assurance can be renderedineffective if the work is done “mechanically”, and QC procedures are ignored. It is also
possible that the staff applies the procedures correctly for new treatment plans, but fails to doso for a modification of the treatment plan (Case 5 in Chapter 4, CP, 2005, NYC-DHMH,
2005).
•

Hospital administrators and heads of radiation therapy departments should provide
a work environment that encourages “working with awareness”, invites
concentration and avoids distraction. They should supervise compliance with QA
procedures, not only for the initial treatment plan but also for treatment
modifications.

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6.9. References
Ash, D., (2007) Lessons from Epinal. Clin Oncol 19:614-615
ASN (2007) Report concerning the radiotherapy incident at the university hospital centre(CHU) in Toulouse – Rangueil Hospital. ASN – Autorité de Sûreté Nucléaire
ASN (2007a) Summary of ASN report n° 2006 ENSTR 019 - IGAS n° RM 2007-015P on the
Epinal radiotherapy accident. G. Wack, F. Lalande, M.D. Seligman
Derreumaux, S., Etard, C., Huet, C., Trompier, F, Clairand, I. Bottollier-Depois, J.-F., Aubert,
B., Gourmelon, P., (2008) Lessons from recent accidents in radiation therapy in France.
Rad Prot Dosim xx
Customer Infromation Bulletin (Nucletron) (2007): Incorrect creation of DRR using GPUbased methods. CIB-OTP 192.047-00.
CP (2005) [Treatment Facility] Incident Evaluation Summary, CP-2005-049 VMS. 1-12.
IAEA International Atomic Energy Agency (1998). Accidental overexposure of radiotherapy
patients in San José, Costa Rica. IAEA Special publication series (IAEA, Vienna).
IAEA International Atomic Energy Agency (2000). Lessons learned from accidental
exposures in radiotherapy. IAEA Safety Report Series 17 (IAEA, Vienna).
IAEA International Atomic Energy Agency (2000a). Absorbed Dose Determination inExternal Beam Radiotherapy: An international Code of Practice for dosimetry based onstandards of absorbed dose to water, IAEA Technical Report Series 398, IAEA, Vienna(2000).
IAEA (2007), Specification and Acceptance Testing of Radiotherapy Treatment Planning
Sytems. IAEA TECDOC-1540. Vienna, Austria.
ICRP (2000) International Commission on Radiological Protection.. Prevention of accidental
exposure of patients undergoing radiation therapy, ICRP Publication 86. Annals of the
ICRP 30(3), (Pergamon Press, Oxford).
Mayles, W.P.M. (2007) The Glasgow incident – a physicist’s reflections. W.P.M. Mayles.
Clin Oncol 19:4-7.
NRC (2007) Gamma knife treatment to wrong side of brain. Event Notification Report 43746.
United States Nuclear Regulatory Commission

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THERAPY TECHNOLOGIESNYC-DHMH (2005) ORH Information Notice 2005-01. Office of Radiological Health, NYC
Department of Health and Mental Hygiene
ROSIS (2007), Report no. xx. Radiation Oncology Safety Information System (ROSIS)
http://www.rosis.info
ROSIS (2008) Report no. xx. Radiation Oncology Safety Information System (ROSIS)
http://www.rosis.info
SFPM (2006) Accident de radiothérapie à Épinal. P.J. Compte. Société Française de Physique
Médicale
SMIR (2006) Unintended overexposure of patient Lisa Norris during radiotherapy treatment
at the Beatson Oncology Centre, Glasgow in January 2006. Report of an investigation by
the Inspector appointed by the Scottish Ministers for The Ionising Radiation (Medical
Exposures) Regulations 2000
Williams, M. V. (2007) Radiotherapy near misses, incidents and errors: radiotherapy incident
in Glasgow. M.V. Williams. Clin Oncol 19:1-3

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MINIMIZING THE RISK OF UNINTENDED EXPOSURE FROM NEWER RADIATIONTHERAPY TECHNOLOGIEAPPENDIX A. SHORT REPORTS ON INCIDENTS FROM THE RADIATIONONCOLOGY SAFETY INFORMATION SYSTEM
A.1. Reports relating to Record and Verify (RV) Systems
ROSIS Incident Report #19: Capturing treatment parameters incorrectly on a treatment unit
Treatment field parameters were transferred from the RV system to the linear accelerator,
including the monitor units (MU) for dynamically wedged (DW) fields. The field size wasintentionally modified manually for a treatment field by using the linear accelerator handcontrol. When the new field size was captured by the RV system, the previous information on
MU for the DW was lost. Two subsequent treatments were given without the DW before theerror was detected.
ROSIS Incident Report #107: RV failure to register a given treatment
A network communication failure occurred between a linear accelerator and an RV systemcausing communications to break down. 30 MU (wedged field) had already been given to thepatient when this failure occurred. To restart communication, the RV system was rebooted,
after which the RV did not acknowledge that part of the treatment of the patient had alreadybeen given. When treatment resumed, the radiation therapists gave the full treatment,
including the previously given 30 MU.
ROSIS Incident Report #116: Incorrect MU registration by RV system
A patient was treated with a field that was open for one part of the treatment and wedged foranother part (using a motorized wedge). When the wedge automatically moved out of the fieldin order for the open field to be delivered, no information was received by the RV system,
thus continuing to register MU for the wedged field. A faulty microswitch stopped the correct
information from being sent to the RV, but the actual treatment had been performed correctly.
ROSIS Incident Report #141: Error in manual set up due to RV not being used because of“millennium bug” problems
An RV system was taken out of clinical use in a hospital because it was considered that there
were problems with millennium bug or year 2000 (Y2K). The system was not replaced on the

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linear accelerator from which this system had been removed, leading to manual treatment set
up and selection of treatment parameters and accessories. A patient treated with twotangential beams for breast cancer noted that the number of wedges used was different fromone day to the next, asking the radiation therapist if there was a reason for this and if it had
any importance. A wedge had been forgotten at the manual set up of one of the tangential
fields of the patient causing an incorrect absorbed dose to be delivered.
ROSIS Incident Report #690: Inadvertent rotational treatment of a patient
During the first treatment of a patient with an electron field, it was noted that the gantry
started to rotate. The prescription was for static treatment, not for a rotational one. An error
had been made when preparing the RV entry of the treatment, where a checkbox had been
accidentally checked for rotational treatment. It was also noted in another report to ROSIS(Incident Report #689) that, for this particular type of RV system, the checkbox for rotational
treatment on the screen was placed near the icon for closing the window after finalized RV
entry, leading to inadvertent activation of rotational treatment.
ROSIS Incident Report #725: Problems in selecting the correct field in the RV system
A patient was treated with overlapping large and small fields, to be delivered every other day.
This was programmed into the treatment schedule of the patient in the RV system. One day
when the patient was treated, it was found that the system allowed the selection and treatmentof both series (large and small fields) on the same day, and that after having irradiated thepatient with the first field, the RV system automatically chose the next field with the lowest
number, which should have been used for treatment the next day. This was noted after a few
MUs had been given. The treatment was interrupted.
A.2. Reports relating to soft wedges on linear accelerators
ROSIS Incident Report #20: Treatment with soft wedges in the wrong direction
During the process of treatment planning, the field names of two tangential breast fields (e.g.
left medial and left lateral tangential fields) were reversed, thus the treatment parameters wereassociated with the wrong field names and vice versa At treatment set up, the fields werecalled up on the RV system. The technologists set up the correct gantry angle, which did notmatch the angle recorded in the RV. Subsequently they overrode the gantry angle on the RV

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record, leaving the remaining parameters from the other field. These parameters included
dynamic wedge data, thus wedge direction were from the other field, i.e., the opposite
direction to that intended.
ROSIS Incident Report #284: Inadvertent loss of wedge code information
Due to a breakdown of a linear accelerator, a patient was moved to another accelerator for asingle fraction. As an inherent part of the design of the RV system, the wedge information in
the RV system was not transferred automatically to the new treatment unit. The wedge code
was manually input properly for the single fraction at the second unit, but when the patient
was transferred back to the original unit, the wedge code was not put in again. As a result, thepatient received treatment without wedges for three fractions before discovery, causing theaccidental delivery of the incorrect absorbed dose and dose distribution.
ROSIS Incident Report #310: Wrong manual transfer into RV system of data on soft wedges
After a patient’s treatment had been re-planned, the wedge code for the dynamic wedges was
not manually entered into the RV system. This meant that both fields of the treatment set up
were used without the intended wedges for three fractions, before discovery.
ROSIS Incident Report #314: Incorrect manual entry of soft wedge direction
The wedge code for a dynamic wedge was entered manually into the RV system. Whenperforming this entry, the wrong wedge direction was chosen, i.e. “Out” instead of “In”,
leading to an incorrect dose distribution in the patient for one fraction before the error was
discovered.
A.3. Reports relating to multileaf collimators on linear accelerators
ROSIS Incident Report #132: Connectivity problems between RV system and MLC unit
For a specific combination of RV system model and linear accelerator model, there was noverification of the MLC configuration of the treatment fields, i.e. the MLC files containing theinformation on the MLC settings for each field had to be opened separately on the linear
accelerator control software in order to set the MLC configuration, without the possibility of
having the correctness of the MLC setting verified. For one field, it was forgotten to open the

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corresponding MLC file and to set the MLC configuration, thus starting to irradiate thepatient without MLC shielding. At the reported instance, the error was detected after a few
MU, and the treatment could be stopped, but the incident report indicates that this problem isa recurring issue.
ROSIS Incident Report #707: Loss of MLC shape after portal imaging
The imaging feature in an RV system was used to make a double exposed portal image,
starting with an open large field in order to make anatomic landmarks more clearly visible in
the portal image. The second exposure of the sequence was intended to be of the actual
treatment field. The MLC settings did however not return to the intended treatment settings.
This was noticed at the treatment unit, and treatment was interrupted.
A.4. Reports relating to Computerized Treatment Planning System (TPS) Tools
ROSIS Incident Report #326: Printout of beam’s eye view (BEV) put together incorrectly
Printouts of beam’s eye view (BEVs) were used to check the shape of the irradiation fields
prior to patient treatment, by placing the scaled printouts on the treatment couch at a certain
distance from the source and comparing with the light field of the corresponding treatment
field. The use of large fields made it necessary to print out the BEV on two sheets of paper
and to put them together in order to cover the whole field. When checking a particular field, it
was noticed that there was no agreement between the BEV and the light field. Wheninvestigating this further, it was found that the two sheets of paper had been put together
incorrectly. It was felt that a contributing factor to this mistake was the insufficient
identification markings on the BEV paper sheets by the system
ROSIS Incident Report #471: Transfer of the wrong digitally reconstructed radiographs(DRR) of the patient.
At the first treatment of a patient, electronic portal images of the treatment fields were taken.
The radiographers on the treatment unit noticed large discrepancies between these images andthe DRR images that were used as reference images of the intended field placement. Further
investigations revealed that DRR images from a different treatment plan of the same patient
had been sent.

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MINIMIZING THE RISK OF UNINTENDED EXPOSURE FROM NEWER RADIATIONTHERAPY TECHNOLOGIEROSIS Incident Report #623: Incorrect labelling of simulator film leading to incorrect BEV
and to incorrect block positioning
A simulator film showing the intended field shape was labelled on the wrong side. This meant
that, when digitized into the TPS, the BEV was mirrored in relation to the intended fieldshape, leading to the incorrect positioning of the lead shielding in the treatment field. Since
the BEV was used to verify the correctness of the block positioning before treatment, thepatient was treated with the block in the incorrect position before the error was discovered
through portal imaging.
A.5. Reports relating to Imaging for Treatment Planning
ROSIS Incident Report #454: CT images associated with the wrong patient when entered into
the TPS
When performing an electronic transfer of CT images, it was necessary to manually associate
the data with a specific patient, since the information of patient identity in the CT data was
not recognized electronically by the treatment planning system. When the CT images of one
patient were transferred into the TPS, the CT data was introduced into the records of another
patient. The error was detected at a later point in the treatment planning process.
A.6. Reports relating to Virtual Simulation
ROSIS Incident Report #161: Problems due to inadvertent energy selection originating in thevirtual simulation process
When performing virtual simulation with a particular system, a field had to be enteredelectronically into the patient CT data in order to set an isocentre in the simulation process.
Thus, a specific photon energy had to be selected for that field, even if the staff at that time in
the process did not know which energy was the most appropriate. As a rule, 6 MV was always
chosen in the clinic, when creating the field for setting the isocentre. When creating thetreatment plan for a pelvic treatment, the planner should change this initial energy to a higher
energy, but for a particular patient this was not done. The treatment was a three-fieldtechnique with two wedged lateral fields. Prior to starting treatment delivery it was noted thatthe energy was too low for a pelvic treatment and the planner made a new plan with a higher

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photon energy. When the energy of the field was changed, the information on the wedges ofthe lateral fields was lost. This was not noticed by the treatment planner. Furthermore, theMUs were already calculated and checked before the change of energy and were not rechecked after the change. The mistake was discovered after the first field was given.
ROSIS Incident Report #573: Different length units in virtual simulation and linear
accelerator
A very small field (6 mm) was virtually simulated. This treatment was not to be calculated by
the computerized TPS, but manually calculated, and therefore it was not electronicallytransferred. When the radiographers on the treatment unit were recording the treatment
parameters into the RV system, they interpreted the field size incorrectly. While the length
unit used in the virtual simulation system was millimetres, the different length units on thelinear accelerator led to the field size being interpreted as 0.6 mm. The mistake wasdiscovered before the start of the treatment.
A.7. References
ROSIS 2008, Radiation Oncology Safety Information System.
Ibbott, G. S., David S. Followill, D.S., Molineu, A. Lowenstein, J.R., Alvarez, P.E., and Joye
E. Roll, J.E. Challenges in credentialing institutions and participants in advanced
technology multi-institutional clinical trials. Int. J. Radiation Oncology Biol. Phys., Vol.
71, No. 1, Supplement, pp. S71–S75, 2008
International Electrotechnical Commission, Radiotherapy Equipment - Co-ordinates,
Movements and Scales, Rep. IEC 61217, IEC, Geneva (1996).
International Atomic Energy Agency, Commissioning and quality assurance of computerizedplanning systems for radiation treatment of cancer, IAEA TRS-430, IAEA, Vienna (2004).
R. Alfonso, R., Andreo, P., Capote, R., Saiful Huq, M., Kilby, W., Kjäll, P., Mackie, T. R.,
Palmans(a), H., Rosser, K., Seuntjens, J., Ullrich, W. and Vatnitsky, S.. A new formalism
for reference dosimetry of small and non-standard fields. Med. Phys. Volume 35, Issue 11,
pp. 5179-5186 (November 2008).